Once you've clicked on the Prospect tab and you've selected a prospect, you'll see the option to add a Tag on the right side of the screen along the main information.
By adding a Tag you can classify/organize your prospects more effectively allowing you to find them easily in the further.
The Tag can be called anything you like.
Remember, once you've applied the Tag to the prospect or prospects you can then filter all prospects by that specific Tag.
For example, if you have prospects that can only fill out an application on a specific date then perhaps the Tag would look like this, "2/24/19 Application"