It's a productivity system that teaches how to take a simple approach to improving your productivity, by encouraging you to focus on forming one productivity-boosting habit at a time. 

• Collect: Get ideas and to-dos out of your brain and onto a list.
• Process: Review your list daily and decide how to act on each item.
• Plan: Pick a few high priority items to accomplish each week and every day.
• Do: Schedule time to accomplish your selected to-dos without interruptions.

To clear your mind and improve focus, get your ideas and to-dos out of your mind and onto a list. 

Documenting to-dos in the moment lessens the likelihood that you'll forget to do something and gives you a master list of to-dos to reference when you're trying to decide where to direct your time.

For your to-do list:

• If the task takes less than 2 minutes to complete, do it immediately and get it off your list.
• If the task takes longer than 2 minutes to complete, set it aside for later.
• If someone else should do the task, send it to that person right away.
• If the task doesn't require any action, delete it immediately to get it off your list.
• If you just need to file the item for reference, put it where it needs to go right away.

...says that when working on big rocks (the most important tasks), you need to minimize distractions as much as possible.

You're more likely to complete small tasks in a single working session, and more likely to make better progress on big tasks/projects that you need to work on over multiple sessions.