A professional copy editor will thoroughly work through your text, making changes that perfect your manuscript by improving accuracy, grammar, punctuation, and spelling, and eliminating redundancies in your text. Related to this, a good editor will also ensure consistency of style, checking that your own personal style remains intact through smaller details such as hyphenation and capitalization.
While the purview of a copy editor will vary from contract to contract, most copy editors will not handle ‘big picture’ issues such as characterization, plot, or structure. That would be the responsibility of a developmental (or content) editor. Copy editing takes a much closer view of a manuscript — focusing on individual sentences and paragraphs.