"The Hourglass (Pilot)" is a magical object seen in the Pilot. The Crystal Gems have been protecting it for a thousand years. It has different atributes and history in "Steven and the Stevens" (see "The Hourglass (Steven and the Stevens)"); this is due to the Pilot not being canon.
The Hourglass allows the user to time travel. The Pilot provides more information on the limitations the Hourglass has, but since it is not canon these limitations might not apply to the Hourglass in "Steven and the Stevens."
In the Pilot, it takes the owner back to a specific time in the past, but only for the same purpose the owner chose to use it. From the moment it activates, only the individual who initially uses it is able to use its power. Steven's initial purpose is to improve his comeback in arguments, so he could only time travel to instants where someone insulted him. In the actual show, there seems to be no limits to what it may do: Steven goes back in time to warn his dad about impending accidents, to form a band, and to run away from and chase after someone.
Steven activates the Hourglass four times in the Pilot. It is activated following Steven's conversation with Lars after leaving the Big Donut; he is offended by Lars' comment on Steven "not being a hot girl." With the Hourglass in his pocket, Steven thinks of a good comeback and wishes he would have said that. The magical item takes him back to that exact moment, but he is embarrassed again after Lars calls him a big joke. Steven repeats the process and leaves. The activation of the Hourglass attracts a huge Electric Skull. Garnet, knowing they might not win, calls Steven a "buttface." He is at first confused by the unforeseen insult, but during the fight realizes that Garnet made the comment so Steven would be able to use the Hourglass and travel back in time in order to make a comeback. Steven knows the attack patterns of the Electric Skull when he returns to the beginning of the fight. They are able to defeat the skull by pinning its multiple arms with the giant metal donut from the roof of Big Donut.
After the fight Steven wishes he had told Pearl he would save all of their lives after she called him a clown, but immediately regrets it as the Hourglass takes him back in time yet again. The angered Steven breaks the Hourglass; however, it still exists in that timeline as Amethyst hasn't given it to him yet.