Where the chord's notes are in the first few frets. This is a map of the notes, not a hand shape to play: Chords on the Guitar shows real shapes.
In a , the 3rd decides whether the chord is major or minor. We can also swap the 3rd of a triad for a different note. The result is no longer two stacked 3rds, and it's neither major nor minor. These are called suspended chords, or sus chords for short, and the two common ones are sus2 and sus4. The name goes after the root: Csus2, Csus4.
In a sus2 chord the 3rd is replaced by the 2nd: the formula is 1 2 5, so Csus2 is C D G. Measured from the root, that's a and a .
With no 3rd, the chord is neither major nor minor; it sounds open and airy, as if waiting to settle. That waiting is why it's called suspended.
In a sus4 chord the 3rd is replaced by the 4th: 1 4 5, so Csus4 is C F G. Measured from the root, that's a Perfect 4th and a Perfect 5th.
Like sus2 it's neither major nor minor. The 4th sounds like it wants to fall back to the 3rd, so sus4 often resolves to the plain major chord (Csus4 → C).
You'll hear sus chords everywhere in pop, rock and folk. Sus2 and sus4 are the suspended chords you'll meet in everyday music.
A fun fact: Csus2 (C D G) and Gsus4 (G C D) have the same notes. Which name fits depends on which note the music treats as the root.
Next: Chords on the Guitar