After participants apply to a study and fill out a screener, you can choose whether to automatically allow them to join the project right away, or manually approve and invite participants.
Manual review (on by default) gives you more control over which pariticpants join your project. You'll need to look over each participant who applies for your study and successfully passes your screener before they're allowed to schedule a time slot or begin a remote task.
Automatic review lets you schedule participants faster, with fewer steps. Participants who successfully pass your screener survey will automatically be able to schedule themselves for time slots or begin your remote task; no need to manually approve and invite particular participants.
On the final screen of the project builder, you can toggle between manual and automatic participant review.
If you change your mind about manual or automatic approval after you launch your project, you can always switch approval types in the project workspace. Head to your project and click "Edit" under the Session details section of the Project overview page.