A Asus4 chord contains the notes A, E, D. It is built from the suspended 4th formula 1 - 4 - 5, with A as the root.
Why does Asus4 sound like it wants to resolve?
The fourth (D) sits a half-step above the major third (C♯) that A major would normally have. Your ear expects that D to fall back to C♯ — and when it does, playing A again feels like an arrival. That built-in pull is what makes sus4 chords such effective decoration.