It means a few things:
MBTI is based off of the psychologist Carl Jung's typology, but uses a tension of polar dualism in BEHAVIOR, not necessarily of mind, to explain personality.
You have Introversion and Extroversion. I am a introvert- don't talk to a whole lot of people outside of work, perfer to be alone or around a few. Extroverts the opposite, they would die being around a lot of people.
Everyone is going to have times when they are more introverted than extroverted, and vice versa. This is a aspect we share.
Sensing Vs Intuition. S Means one is more prone to the senses in reference to their I/E state to indicate something is going on, so if your a IS means your introverted and sensing, your going to pick up on stimuli faster consciously and with more emphasis in the long term to concrete, real actions coming in from the real world. It's opposite is a Intuitive, who figures stuff out more often. Instead of joining in on a discussion 'where is it' with the other Sensing people trying to figure out who knows what, the intuitive figures out where that thing likely will be. Sometimes you gotta tap a intuitive on the shoulder to get their attention, othertimes they'll know your about to tap them on the shoulder and will answer your question before you ever say it, cause they can sometimes figure this stuff out.
Someone's who in a situation and their initial impulse is to think things through like Spock from startrek is a thinker. Someone who is like 'oh fuck no..... nooooooooo! is probably a feeler, at least at that moment.
Judgment and Perceiving..... it's the willingness to come to a conclusion, such as 'this is that, that is this, I am done with this thought, for I have figured this out. That is judgment. Perceiving is prolongation of the investigation, and not really coming to a conclusion.
So, not only do you have the tension of opposites in a line, such as INTJ in my case, but you have levels to it. IN supposes ES is my secondary fall back. I can be walking around, figuring stuff out, and can rapidly fall back in on a ES situation where I see everyone blathering on, confused as fuck, and quickly insert myself into that situation. As my primary is NT, and I am also Judgmental, I can very, very, very rapidly come to a conclusion and express it while most people are still introducing themselves. Think Sherlock Holmes.... a good INTJ.
Now, this is a typology not meant FOR THE MIND in terms of speaking about it's biological parameters, but what we can assert is happening behaviorally, and make it predictable, and notice personality traits stabilize in it.... but when it's stabilize, we notice there are a few variations of personality type per type that are similar and yet differ.... but even the differences are stable in that people will differ in a very similar way.
Some have grouped INTJ, ENTJ, and ISTP together in the Supplementry Motor Area of the brain, naming it a personality matrix of sorts, where a person will cycle through those aspects but not really others...... but then note a different but very similar kind of INTJ exists nearby in the Right Hemisphere, and that they don't cycle through that set, but other people do. It took a while, but when the cranial nerves were mapped out over what was known of the brain, we found we could hook up those's INTJ regions from one end of the neuron to the other passing through two regions, and found another usually passing back the otherway which was it's opposite in one of the letters.... and that the personality behavior remained stable. This is a feedback loop, and it maps our conscious and unconscious elements.
MBTI also has a theory of sexuality. In terms of predicting fetishes and style of intimacy, it's dead on. In terms of predicting life long mates, it sucks. It automatically assumes I as a INTJ would want a opposite, a ESFP female, otherwise known as a bimbo party animal. Though sexually we might click at times if I am in the right mood, usually I would want nothing to do with such a woman. As I pointed out, MBTI is a behavior system, but didn't in the beginning delve into why things were as they were. I might want a woman who's my opposite in terms of brain regions where I have a weakness and where I am attracted to strength that isn't as perdicted as in letter opposites, but location of brain regions.
However, more often than not, it's good at predicting what your mate's personality type will be. Your ISFP, so walker in it would be a ENTJ. He's a INFJ though (borderline ENTJ though none the less, if he matures- his love for conspiracy theories is a disease of the NTJs.... know too many into that, even though I am not, because I have a better sense of ES as I am not hateful of others, whereas his paralell is IS for his secondary and will always fall back to being hurt by others), so whatever.
The modern usage of MBTI varies. Sometimes the old style will be used on dating sites. Othertimes it will be used in a pretty advanced theory of mind. There are several variations of it..... in a swedish version I came off as a INTP, and I emailed the team, and told them they messed up on my typology..... they then asked my if I typed myself, and I said yes, and they pointed out they were psychologists who knew the answer better than I could.... and I fucking unleashed on them and in the end it was decided maybe I was indeed a bit judgmental after showing how idiotic their assumptions were in formatting the questionnaire in not picking up on INTJ quirks, as well as misrouting a few judgmental passages in the brain. Swedes have a disturbing history of fucking around with with the lay psychological movement, and INTJs have a disturbing history of collapsing presumptuous and backwards systems.... so in the end, it was inevitable I would react a tad bit hostile and go on the offensive.
It's like Prussia calling itself that name when living so close to Russia.... you know it's gonna piss the Russians off.