Editing typically falls into two categories: line editing and copy editing.
Line editing means checking for spelling errors, correct usage, making sure verb tenses agree and that all grammar is correct. I proofread billboards. Can’t help it.
Copy editing is more global, asking questions about audience and readability, voice, the accuracy of the information. It also addresses whether or not the content accomplishes what you want it to: impart information, provide step-by-step instructions, pique a reader’s interest.