It's not a really long read, and it's worth reading it cover to cover, even if you don't follow its advice to the letter.

That said, essentially:

Keep a daily list of all the next actions you want to accomplish, separated by context (e.g., at work, at home, at grocery store)

Break down large projects into next actions. Identify the next thing you need to do to move it forward.

Review your projects and next actions weekly (or more often) to make sure you're not missing anything.

There's other stuff too, but mostly GTD recommends that you write everything down and review it often. Your lists are a way to offload the stuff in your head, so that you can focus on what's right in front of you and not have that nagging feeling that there's something you forgot to do or something more important you should be doing.

It definitely changed the way I approach my day at work, and I feel like I'm able to accomplish a lot more without feeling exhausted at the end of the week.