Level 0 implies that no finishing of any type has been done. At this level, drywall is simply fastened to the walls or ceiling.

Level 1 means that drywall joint tape has been embedded in joint compound, but nothing further has been done. 

Level 2 means that you have skimmed a thin coat of joint compound over the tape and covered the drywall screw holes. 

At Level 3, finishers apply a coat of joint compound to the tape and screws. Walls that will receive a texture finish. 

Level 4 is the classic drywall finish. Here, you apply another coat of joint compound to the tape and screws and sand the dried compound. 

Level 5 is the highest possible level of drywall finishing involves applying a skim coat.

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