The right slash / or diagonal line written above the staff where chord symbols occur is used to indicate a beat during which the most recent chord symbol is understood to continue. It is used to help make uneven harmonic rhythms more readable. For example, if written above a measure of standard time, "C / F G" would mean that the C chord symbol lasts two beats while F and G last one beat each. The slash is separated from the surrounding chord symbols so as not to be confused with the chord-over-a-bass-note notation that also uses a slash.
For chord abbreviations, the right slash indicates the bass note if other than the root. It is usually written with the complete chord name, and, after the slash symbol, the desired bass note. For example, the symbol C/G would mean that the chord to play is a C major triad with a G as the bass note, leading to the following notes: G C E (commonly known as the 2nd inversion C major triad).
A right slash surrounded by two dots