Watch the Building Suspended Chord Triads online guitar lesson by Rob Garland from Chord Navigator: CAGED Triads
A suspended chord contains the root and 5th, but the 3rd is replaced by either the 4th (sus4) or the 2nd (sus2). These chords sound, funny enough, "suspended" or unresolved as the 4th or 2nd wants to resolve back to the 3rd. They are often used to create melody lines and in jazz it is common for them not to resolve at all. A typical chord progression with a suspended chord would be D-Dsus2-Dsus4-D.
I'll use a C chord (C-A-G-E-D "A" shape) as an example played on just the D-G-B strings. To make the chord Csus2 replace the 3rd, the note E on the B string, with the note D. To make the chord Csus4 replace the 3rd with the 4th, the note F on the B string. Note: If you just see the word "sus" written after a chord name, it's implying sus4.