why does it change when I put a different letter in %?

Question

I just recently start learning codes and I see in the examples than all the letters that are being used with % changes. I try to change a letter to see if its just something you write to be filled with the next value, but when i change it start trowing numbers symbols or simple "no output".
Is that because all the letter used with % have their own funtions?
If so, how can you know what letter use with % in any case?

Answer 1

thanks you so much!