Copyediting conforms the text (the “copy”) to standard rules and conventions of grammar, syntax, word usage, spelling, and punctuation, as well as checking for consistency of style and formatting. It’s a basic edit, aimed at correcting the copy for plain errors rather than revising it for cogency or effectiveness; the latter is the province of substantive editing.
The copyediting service is appropriate for generally well written documents that have gone through at least one round of substantive editing and just need a professional editor with fresh eyes to review the work and give it a final polish.
Copyediting includes proofreading. The main difference between the two is that proofreading is mostly concerned with typing/input errors left over in already copyedited documents, whereas copyediting deals with errors in the writing itself, albeit at a basic level.