The Inbox explained
The Inbox is where you temporary put new ideas and thoughts. You should sort the Inbox as often as possible, choosing what you want to do with each item.
If you're not sure wether you want to commit to a task, you may move it to Someday for later reference. If you decide not to commit to a task, you may drop it. Any task that actually consists of more than one task should be converted to a project.