This setting controls whether the user is considered to be on probation as a possible spammer. Newly self-registered users start out with a certain number of "probation points" on their account. While they have any probation points, they are considered a probationary user and cannot create public pages in their portfolio or post links or images to potentially public content such as comments, wall posts, or forum posts.
Each time a probationary user makes a post in a forum, and the post is replied to by a non-probationary user, the probationary user loses one probation point. When the number of points reaches zero, they are no longer on probation.
Probation points have no effect on users who have staff or administrator status.