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