Taping and mudding hides the seams between drywall sheets so the wall looks like one continuous surface. It's a learnable skill — the secret is thin coats and patience, not piling on compound.
First coat: tape
Spread a thin layer of joint compound over the seam, press paper tape into it, and smooth out the excess with a taping knife so the tape beds flat with no bubbles. For inside corners, crease the tape down the middle.
Second and third coats
Once dry, apply a wider coat over the tape, feathering the edges out so the buildup blends into the wall. The third coat is wider still and thin. Each coat should be smooth — fix problems now, not by sanding later.
Sand and prime
After the final coat dries, sand lightly to remove ridges, wearing a dust mask. Wipe off dust and prime before painting. Good mudding means very little sanding.