Here's why:
* The F clef (also known as the bass clef) is placed on the fourth line from the bottom of the staff.
* The line where the F clef sits is always F.
* The spaces above and below that line represent the notes G, A, B, C, D, and E, ascending and descending respectively.
So, if you see an F clef, you know that the line the clef sits on is F, and you can work out the notes from there.