Its a voting mechanism. Those votes determine what links appears on the front page as almost nobody browses the new submissions. Also, voting ranks comments.

Its a voting mechanism. Those votes determine what links appears on the front page as almost nobody browses the new submissions. Also, voting ranks comments. As such this is a means of shifting priority based upon agreement. When a negative vote is cast it becomes one of two means to enforce a heckler's veto. The other form of heckler's veto is by click on the link to "flag" a comment or submission as after an item is flagged 4 times it is hidden from everybody. Notice that those two mechanisms are completely unrelated in their functionality even though the results are similar. The problem with voting mechanisms is abuse. They are extremely good within small communities, but once a venue grows past a certain size it becomes a means for insecure people to impose echo chambers.