Bumping a thread moves it to the top of a board's list of threads, as though you had made a reply.
To bump a thread while you're viewing it, click on the Actions dropdown at the top of the thread. Select Moderate, and then select Bump.
To bump multiple threads to the top of the thread listing at once, visit one of your boards and select the checkbox next to each thread you want to bump to the top. Click on the Actions dropdown, select Moderate, and then select Bump. This will affect every thread you selected.