A Time Turner is a Fictional Object from the Harry Potter Universe
A Time Turner is described as an hourglass on a chain that allows the wearer to travel back in time. Each turn of the hourglass equals one hour back in time. One important rule, however, is to never let their past self see the time travelling version of themself.
In Harry, Ron and Hermione's third year, Hermione was given a Time Turner by Professor McGonagall, as she had chosen more classes than she would be able to attend in normal time. With the Time Turner, Hermione was then able to travel back in time and attend the classes that she would otherwise miss. Professor McGonagall reminded her that her past self could never see her future self. Hermione kept her Time Turner a secret from Harry and Ron until the end of the year when herself and Harry travelled back to save Buckbeak and Sirius Black. Afterwards, Hermione dropped a few of her classes and returned her Time Turner, saying it was too much hassle.