Time-Turners are used to send the person or persons who wear it travelling through time. For every hour travelled, the Time-Turner must be turned once. Time-Turners are potentially dangerous contraptions, and thus are protected by the Ministry of Magic, who only give them out in very special circumstances. Professor McGonagall had to write many letters to the Ministry of Magic before they allowed Hermione to have one. The consequences of using Time-Turners can be very serious; many wizards who have meddled with time have ended up killing their past or future selves by mistake, not understanding how or why they are able to see themselves.