A Time-Turner is a device used for time travel. It is a special timepiece which resembles an hourglass on a necklace.
The Ministry of Magic encased an Hour-Reversal Charm in the time turners they created, for added stability. The number of times one turns the hourglass corresponds to the number of hours one travels back in time. However, they can only stay in the past for five hours at a time, without the possibility of serious harm to the traveller or to time itself.