They may be required to correct spelling, grammar and layout (proofreading) or check content, impose consist styles and reword or rewrite (copy-editing). This depends on the project, the employer/client and their own specialist experience. Many copy-editors/proofreaders perform both functions in tandem.
They're employed by publishers, businesses and public bodies but increasingly on a freelance basis, so it's common for them to have several clients at the same time. Many people retrain for editorial work as a second or third career.