The Power Query M function reference includes articles for each of the over 700 functions. These reference articles are auto-generated from in-product help.
If you're familiar with using Excel functions (and of course you should be) then you'll know that a function typically takes some input and gives you an output.
Hidden inside both Excel and Power BI is a secret Power Query function list that you can bring up while working in the Power Query editor window…that’s if you know the magic word.
Power Query has over 600 native functions and the Power Query team keeps adding more and more.
I wouldn’t recommend memorizing them, but you do need to understand the concept of parameters and arguments in order to understand what functions are.