Joint compound, usually called mud, is the paste used to finish drywall seams and cover fasteners. Sanded smooth, it turns a wall of drywall panels into one continuous surface ready for paint. Skipped or rushed mud work shows up in the final finish, so we treat it as real prep work.
Drywall mud goes down in layers, each one feathered wider than the last so the seam disappears into the wall. Sanding between coats catches ridges and bumps, and a well-mudded wall makes every coat of paint that follows look better.