To proofread is to look over a piece of writing for errors, watching out for spelling mistakes and grammar problems. If you don’t proofread your work, you might write “ham” instead of “harm” and confuse the heck out of your reader.
By the time you proofread a piece of writing, it should be almost done. It’s the very last step of the writing process after drafting, editing, and revising. To proofread is to look at the little details of a piece of writing, such as spelling, word choice, formatting, and sentence structure. When you proofread, you look for errors and fix them. You can proofread your own work, but sometimes it’s good to get someone else to proofread what you've written.