Positioned here, it makes the second line from the top of the staff F below middle C, and is called a "bass clef." This clef appears nearly as often as the treble clef, especially in choral music, where it represents the bass and baritone voices. Middle C is the 1st ledger line above the stave here. The shape of the clef comes from a stylised upper-case-F (which used to be written the reverse of the modern F)