Peer voting on questions and answers is the primary way users gain reputation, and also how many items are sorted to the top.
Voting is a mechanic on this site used to identify if a question, answer or comment are helpful or not. When a post gets up-voted, the author receives reputation as an indication that his post was helpful to someone. When your post gets downvoted, you lose some reputation as a notification to try harder or fix some inaccuracies on your post.
Upvoting requires 15 reputation, and downvoting requires 125 reputation. When you downvote, you should put a comment why you downvote, and downvoting on answers takes 1 reputation from you, so use it wisely. On the meta, upvoting on feature requests also indicates that you agree with the suggestion.
If you vote and leave it for some time, your vote gets locked. This means that you cannot change your vote, unless the question is edited.