When I first started visiting MTBI forums I felt exactly like you do. I kept seing Ni and Fe everywhere and had no idea what people were talking about. I tried researching what they were talking about it but it just didn't make sense. And then one day I read an article (on the forum I was frequenting at the time) comparing the different functions to animals. For some reason that article made everything click for me.
So, I'll try to explain it how I would have explained it to myself if I could jump in a time machine and go explain stuff to myself
The functions come from Feeling, Thinking, Intuiting and Sensing...and then introverted and extroverted versions of each. So in total there are 8 functions. Fe, Fi, Ti, Te, Ni, Ne, Si, Se.
Each type has a group of four functions. Our dominant function, auxiliary function, tertiary function and inferior function.
Dominant is the one you use all the time. Auxiliary is the one that kinda helps out the first one you use it a lot too. Tertiary develops as you get older. Inferior, is your weakness.
As an INFJ your functions are :
Ni (Introverted Intuition) (dominant)
Fe (Extroverted Feeling) (auxiliary)
Ti (Introverted Thinking) (tertiary)
Se (Extroverted Sensing) (inferior)
Check this link
http://typelogic.com/infj.html
Scroll down to the part where it says Functional Analysis. It breaks down your four funtions and what they mean.
Hope that helps.
If you have any more questions feel free to pm me and I will share what i know.