If you press the "f" key in normal mode, Vim will move the cursor forward to whatever character you input after "f" is pressed. As an example, consider the following line:
a quick brown fox
If the cursor was at the absolute beginning of the line, and you pressed "fb" in normal mode, Vim would move the cursor so that it was positioned over the "b" in "brown".
If you press "F", Vim will move the cursor backwards instead of forward. Given the previous sentence, if pressed "Fq", and the cursor was at the end of the line, it would move to the "q" in "quick".