As you know Featured ads always shown at the top of the page/list till expiry so Bump Ad option is best for regular/free ads.
Bump Ad: When a user post an ad, Then after some time/days that ad will be goes on second page or down, And that user want to show that ad again on top of the list, Then User can use Bump Ad option, So Bump means that will be update to Current date & time, And again That ad will be come at the top of this list.
When more ads will comes then that ad will again goes to down.
Bump Ad mean , User can show his/her ad at the top of the list when ad will be goes to down.
For more help you can watch this video.
Thanks