Voting to Close is the process by which users can vote to close a question. You gain the ability to cast close votes when you reach 3,000 Reputation on graduated sites, and a lesser amount is needed on beta sites.
Voting to Close is the process by which users can vote to close a question. You gain the ability to cast close votes when you reach 3,000 reputation on graduated sites. The threshold is 500 reputation for public beta sites and 1 reputation on private beta sites. On Area 51, example questions can be closed with 150 reputation. Proposals can only be closed by moderators.
You can also view and cast close votes on your own questions if you have at least 250 reputation on graduated and public beta sites, or 1 reputation on private betas.
Questions should be closed by casting close votes if:
they are sufficiently similar to existing questions and would be answered identically to them
they are unclear, too broad, or otherwise problematic to identifying the problem in a way that can be properly addressed by answerers
they are sufficiently off-topic, as outlined in the help center
It takes 5 votes to close a question. Gold tag badge holders can single-handedly close and reopen questions as duplicates (source), and diamond moderators can instantly close or reopen any question.
For more information on the closing system, see cast close and reopen votes and What is a “closed” or “duplicate” question?.