Suspended (sus) chords
In harmony there's a concept called 'suspension', which is the delay of a portion of a chord change. If you're moving from an F chord to a C chord, the movement of the notes looks something like this:
| C | stays | C |
| A | moves down to | G |
| F | moves down to | E |
In a suspension, that chord change takes place in two steps:
| C | stays | C | stays | C |
| A | moves down to | G | and stays | G |
| F | stays | F | then moves down to | E |
The result can be a bit 'smoother' than the simple F-C change. The chord in between is seen as a C chord, but with the fourth (F) replacing the third (E). The result is called a C suspended chord, or Csus. Some charts use Csus4 to make it clear that the fourth is included, but that's not really needed - and suspended chords always use the fourth. If you see a notation Csus2 (meaning C-D-G), the chord should probably be named Gsus4 (G-C-D).
Here are the common suspended chord fingerings:
