The Ultimate Marvel Universe was a new imprint of comics that took all of the Marvel properties and created a new universe independent of the original 616 main continuity.
Primarily, it allowed writers and artists to reinvent famous characters such as Captain America and Wolverine without having to worry about 40 years of previous continuity, as well as serving as a way to introduce new readers and younger audiences to comics who weren't interested in braving the perils of back-issue bins to figure out an unending list of retcons.
The Ultimate Marvel Universe ultimately came to an end in 2015's Secret Wars storyline, where the most popular characters were moved from the Ultimate Universe into the main continuity. While the Ultimate imprint did have its issues across its fifteen year lifetime, it was also home to some of the greatest comics Marvel ever printed - one of which was Ultimate Spider-Man.
Peter Parker's Spider-Man received one hundred and sixty issues in the Ultimate Universe while Miles Morales would get twenty eight before joining the main continuity. In this time writer Brian Micheal Bendis - together with an array of talented artists like Mark Bagley and Sara Pichelli - was able to tell one of the greatest Spider-Man stories ever, taking the best of Spider-Man and using it to tell a truly satisfying story about power and responsibility.