Four functions, four slots
In traditional Jungian systems and MBTI the cognitive function stack is made up of four cognitive functions. There are quite a few rules when it comes to the function stack but the simplified way of approaching it is to look at how a function typically presents itself due to its slot. There are patterns within the stack itself that can also help you understand the psyche from a type perspective.
The first function slot is the minds auto pilot. Our natural way of processing information. Confidence, ease of mind, and relaxation are what typically come to mind when thinking of this function. For a Ti dominant type, this might look like learning or solving complex problems. For an Se dominant type this might look like them engaging in a high energy activity.
The second function is one that gets too much praise or attention in my opinion. A lot of people associate it with proficiency and even mental comfort but what they tend to fail to remember is that the second function exists in the opposite energy attitude. So for example, if you are an introverted type the second function is extroverted. This means that despite being helpful and often easy to use, it is inherently a draining process that one must eventually come back from to recover. For an INTJ, they might feel that it is easy to take charge and organize but at the end of the day they are an introvert and they need to go back to introversion to allow the psyche to release tension and relax. This function is often called the "tool" function because it is brought to attention when needed, but then sheathed or stored away when not.
The third function is an odd one. It exists in the same energy as the dominant attitude BUT it is not the dominant function. This means that is can often be "pushed back" behind it. Imagine a short person standing behind a taller one. They are still there, they can just be hard to see. Just like someone who may have something to prove, sometimes the third function can want to "prove" itself, especially in the case of extroverts. Introverts on the other hand tend to see this function as a secret or hidden desire that they wish to keep to themselves, a private comfort or identity.
The second and third function do not have as much conflict as the first and fourth. This is because the first pulls so strongly into consciousness that we become unaware of our fourth to some degree. This same degree of consciousness and pull is not as drastic with the center two functions and thus we often find stability and compromise more often between them.
Finally we have the fourth function. It exists as the shadow to our dominant. Whereas we pay a lot of attention to our dominant, our last is often a blindspot. It can be difficult to pay attention to and when we need to it often feels exceptionally stressful or taxing. It can seem like others who value this function more highly than you will be nitpicking or criticizing you while you likely feel it is not worth the effort for the returns. The secret to conquering this function is awareness, not mastery. Don't try to pretend to be someone you are not, but instead be aware of how thing things that you struggle with might be holding you back.
First + third function = comfort and identity.
Second + fourth = Persona and aspiration.
Second + third = balance and compromise between the two.