A preventer is a line that prevents the boom from accidentally jibing when on a run. The preventer, of course, will have to be slacked or untied before intentionally jibing, or before heading up.
A preventer can be a single line run from a midpoint on the boom to a suitable point on the rail or the foredeck. Often, the boom vang is used as a preventer by detaching it from the base of the mast and attaching it to the rail.
A more complex solution is to run two lines from the boom to blocks on the foredeck, and then back to the cockpit. This allows the crew to slack and tension the two preventers as needed without leaving the safety of the cockpit.