When doing a home renovation project you will most likely be installing some new drywall (sheetrock) and will need to fill in the seams. This is done with drywall joint compound (mud) and tape.
Finishing drywall is fairly simple once you get the hang of it. You start by placing a small amount of mud on the seam, then place the tape over it, press the tape tight to the seam and remove the excess.
You will need a couple of different width blades, as you need to put on a few coats, making each one wider than the last, so the joint “feathers” out and becomes almost flat so you do not see it after it is painted.