They will charge you for any damage to the unit. It could include something as small as a nick in the wall to a stain on the carpet. If anything is missing from the unit since the time you moved in, you could be held liable for that too. That could include light fixtures, curtains, blinds etc.
The landlord can charge you for the replacement item and the labor to have it installed, patched or repaired. If you believe that the charges are excessive, you can contest the charges but you would have to check what the law allows in your state.