This is a flat sign.
A flat sign written before a note on the staff means that you play the note that is one fret lower than the note written. For example, the note written on the staff above is called B flat (Bb), and is played on the third fret of the third string. When a flat sign is written on the staff, it is always written before a note.
To play the Bb note, place your third finger just behind the third fret of the third string.
Rhythm Pattern
C7
To play the C7 chord, use the four fingers of your left hand as shown in the diagram, and strum all six strings.
Notice that the C7 chord is just a C chord with the fourth finger added.