It means you raise the third to a perfect fourth or lower it to a major second.

C chord: C E G
Csus4: C F G
Csus2: C D G

Often, a sus4 chord will resolve to a major chord (Csus4 to C) and sus2's will resolve to a minor chord (Csus2 to Cm) but this is hardly a definate rule. However, it is a much of a rule as it can be that a sus chord must resolve to some other chord.
'sus' means suspended in case you weren't sure.

...and cos there is no 3rd, they cannot be classed as maj or min...which is kinda cool.
Yeah, 'sus' stands for suspended, and it is replacing the major or minor third with either a major second or a perfect fourth; as with most chords, these also have their own unique sound.