The functions and function model are what makes up your operating system. The differences may be small between types, such as the ENTJ and INTJ where the only thing that differs is the order of their functions. Or they may be rather large for instance an INTP and ENTJ, where the functions are identically flipped.
Your operating system consists of 8 such functions put in a hierarchy that decides your preference in using them, how confident you feel in each one, and how other people will perceive you when you are engaging in them.
For instance, we’re all usually comfortable and confident when using our leading function, but less comfortable when using our inferior function.
This can be likened to a car, with a father, a mother, a 10 year old and a 4 year old.
If dad (dominant) is driving, everything is fine and the trip is going as planned. If dad gets tired and mom (auxiliary) takes over, the trip is still going to be progressing quite nicely. However, if for some reason the 10 year old (tertiary) is suddenly driving the car, the trip becomes more unsteady, less safe and less comfortable for everyone involved and the same degradation of the trip quality happens when the 4 year old (inferior) takes control.