Proofreading and editing are often words used together however they are distinct stages in reviewing your work.
Editing - re-reading your work to make sure your writing is clear and concise. This means checking your assignment overall to ensure your argument is clear; taking a narrower approach and reading each paragraph individually; and ensuring the structure of each sentences allows your point to be made clearly. It is reading your work to ensure that it makes sense to your reader.
Proofreading - ensuring that your work adheres to expected academic standards and is formatted appropriately for the assignment type. This stage focuses on your spelling, punctuation and grammar rather than the content of the assignment.