Joint compound, universally called mud, is the gypsum-based paste used to embed tape over drywall seams, cover screw heads and corner bead, and feather everything into a flat, paintable surface. It comes in two families. Ready-mixed (premixed) compound is sold wet in pails and boxes and dries by evaporation; it includes all-purpose, lightweight and topping versions. Setting-type compound (“hot mud”) is a powder mixed with water that hardens by chemical reaction in a stated time — 5, 20, 45 or 90 minutes — so it can be recoated the same day and shrinks less.
A standard Level 4 finish takes three coats: a tape (embedding) coat, a fill coat and a finish coat, each feathered wider than the last — roughly 6”, 10” and 12”+ across a flat seam. Pros often bed tape with all-purpose or setting compound and finish with a lightweight topping that sands more easily. Mesh tape requires setting-type compound for the first coat; paper tape works with any.
For estimating, USG guidance for ready-mixed all-purpose compound is about 9.4 gallons per 1,000 sq ft of board — roughly two 4.5-gallon pails — plus a rule-of-thumb 370 ft of paper tape. A Level 5 skim coat over the entire surface can nearly double the mud.