So the timeline of the new Trek movies starts with the villain Nero going back in time. When he gets there, he destroys a ship called the USS Kelvin. That ship was not destroyed in the original Star Trek timeline, so the new timeline is called that because the Kelvin's destruction was the first major point of divergence that lead to the other changes in that timeline

Canonically, Nero started in the OG/main timeline, in the 24th century (Some time after DS9/Voyager, or the movies with Picard like nemisis.) He was the captain of a mining ship, then there was a star that went kaboom; and destroyed the romulan homeworld.

Old Spock was working as an ambassador and was bringing Magic Red Stuff™ to stop the supernova by creating a black hole or something. In any case, they arrived way too late, Romulus was destroyed anyway, the supernova was stopped (not how physics works, but okay,); and the Jellyfish (Old-Spock's ship) and the Narada (Nero's ship) were sucked into a black hole. the black hole sent them back in time; the Narada to an earlier point, Old-Spock to a not-so-recent timeline.

The USS Kelvin was destroyed after Spock came back, responding to a distress single and getting the shit kicked out it. the Kelvin, incidentally, was commanded by George Kirk, James T. Kirk's father. (in the OG timeline, George was present in Kirk's life, etc. Which completely changed Kirk's nature.)

Nero's and Spock's presence irrevocably altered that timeline, creating a different universe. Because the Kelvin was where things started diverging significantly, and because Fans are fairly simple minded... (sorry.) ... this timeline is now called the 'Kelvin timeline'

Out of universe, Abrams found that the OG lore background was far to restricting, and didn't allow him his much-abused lenses-flares. so he threw a tantrum and created a new timeline that would allow him his, "creative" liberties.

Others have already given you an answer, but for future reference, whenever you need to find something out about star trek there's memory alpha. It's basically the Trek encylopedia and is very detailed. It's the trek equivalent of tv tropes, so can be a dangerous time suck. So in this case:

The alternate reality was a new reality created on Friday, January 4, 2233 (stardate 2233.04) when a temporal incursion caused by time travel of the Narada, a Romulan mining vessel from the year 2387, disrupted the time continuum of the prime universe. Accidentally traveling back to that point in time, Nero, the Narada's captain, attacked the USS Kelvin resulting in the deaths of several crew members, including George Kirk and Richard Robau, and the destruction of the Kelvin itself. Spock arrived to the alternate reality in 2258 and was captured by Nero, who used red matter to destroy Vulcan. However, Nero's attacks united the crew of the USS Enterprise, who foiled his attempt to destroy Earth. ... Scans and telemetry of the 24th century Narada, taken by the Kelvin, were brought back to Starfleet by the survivors on the Kelvin's shuttles. Therefore, Starfleet's development and construction plans were slightly altered, making everything potentially more advanced, slightly ahead of schedule. ... co-writer Simon Pegg had a different view of how the alternate reality diverges from the prime timeline from Orci and Kurtzman, believing events before 2233 were different too ... "Spock's incursion from the Prime Universe created a multidimensional reality shift. The rift in space/time created an entirely new reality in all directions, top to bottom, from the Big Bang to the end of everything. As such this reality was, is and always will be subtly different from the Prime Universe."

https://memory-alpha.fandom.com/wiki/Alternate_reality

(https://memory-beta.fandom.com is also extensive, but is for non-canon works)

It’s called the Fahrenheit timeline in North America,

Because the first thing Nero does when he arrives back in time is destroy the USS Kelvin (the ship where Kirk's father is killed), and thus setting that universe off onto its new timeline. It could have been called anything, but someone decided on that one thing.

Apparently one of the first changed events in the timeline was that the ship that went back in time attacked the USS Kelvin.

https://memory-beta.fandom.com/wiki/Kelvin_timeline#Nomenclature

Because JJ Abrams uses the name Kelvin in a lot of his works. I think it was his grandfather or something. But in Star Trek specifically, it is the name of the ship around which the events that the divergent timeline started.