An accepted manuscript will be send to a type setting service and you will receive the typeset proofs of your manuscript. Sometimes very important things like tables, figures, headings are missing. But also minor changes might be required.
Nowadays PDF proofs allow you to work electronically on these proofs. But sometimes extensive changes are needed which can not be insert in the proof itself and additional handwritten feedback is required. For such purposes there are “codes” you should use for such corrections. You find a list of such codes here (GERMAN) or here (ENGLISH).
I wish you many proofs with a low number of errors.