A Gsus4 chord contains the notes G, D, C. It is built from the suspended 4th formula 1 - 4 - 5, with G as the root.
When would you use Gsus4 in a progression?
Most often as a decoration on a held G — strum G, lift into Gsus4 for a beat, then fall back to G. The brief suspension adds momentum to a bar that would otherwise sit still, which is why it turns up so often in strummed acoustic parts built around G.