Sus4 (or just sus) stands for „suspended 4th“. The 3rd of a major or a minor chord is suspended and replaced by a perfect 4th. On the guitar you can easily get this chord by taking the chord shape of a major chord and moving the major 3rd (can occur more than once in a chord shape) up by just one fret (a half-step).
Consequently a C major chord has the tones C (1), E (3) and G (5),
a Csus4 chord has the tones C (1), F (4) and G (5).
Instead of Csus4 you can just write Csus (sus = sus4).
Besides the sus4 chord there's also a sus2 chord that has a major 2nd (a whole step respectively two frets higher than a root tone) instead of a 3rd: Csus2 has the tones C (1), D (4) and G (5).
Attention: you can nearly always change a sus4 into a sus2 chord, but don't just do it the other way round. The sus2 chord on your lead sheet may just be meant as an add9 chord without the 3rd being played. In that case letting ring a 4th is like stumbling into a wasp's nest. So carefully ask your ears first!
To make one or more bars of a single major chord more interesting, you can switch back and forth between the sus and the major variant of that chord or you play sus in the first half of the bar and then major.