Ring | Dragon Ball | Goku Time Ring

The Time Rings (時ときの指ゆび輪わ Toki no Yubiwa) are accessories of the Supreme Kai which enable them to travel through time. They are normally used to observe future events in their main timeline. The Time Rings have several special abilities, unlike the Time Machines created by Capsule Corporation, the Time Ring allows for "natural" time travel, and so does not create alternate timelines when used. Additionally, in the anime a wearer of a Time Ring will be granted acausality, and so even if their personal past were to be altered by someone or something - they would not be affected.

It had been said that the Time Rings were used and only worn by the Supreme Kais who often used them to travel through time. Having a Potara earring is required in order for anyone to use the Time Ring, as explained by Gowasu.

As demonstrated by Gowasu and Zamasu, in order for one to time travel using a Time Ring, the one not wearing the ring must touch it with their fist, then both must say the number of years they wish to travel.

Alternatively, as demonstrated by Goku Black, when using a nearby distortion in time to time travel, the ring shimmers briefly before producing a thin beam of light that cleaves a portal in time (as though literally cutting through the space-time continuum) that can be used for travel (even to the past) until the distortion is corrected, at which point the wearer is forcibly returned to the era they just vacated.

Dragon Ball Z (from Dragon Ball Wiki)
Dragon Ball Z follows the adventures of the adult Goku who, along with his companions, defends the earth against an assortment of villains ranging from intergalactic space fighters and conquerors, unnaturally powerful androids and near indestructible magical creatures. While the original Dragon Ball anime followed Goku through childhood into adulthood, Dragon Ball Z is a continuation of his adulthood life, but at the same time parallels the maturation of his son, Gohan, as well as other characters from Dragon Ball and more. The separation between the series is also significant as the latter series takes on a more dramatic and serious tone. The anime also features characters, situations and back-stories not present in the original manga.