At Admin Panel -> Settings -> Switches, the "submission history in profile" switch enables a spot on the "view profile" page which shows short previews of the member's most recent topics and posts (which can be clicked to view the full topic details / thread).

When the switch is off, the profile simply shows a link to a search results page to show all topics or posts by the member.

When you have an extremely large database (hundreds of thousands of topics or posts) this can slow down the view profile page and increase server load, so you may want to turn it of. For 99% of users it will have no noticable impact on speed/load.