What Are Gens?
"Gens" or "Generations" are the way pony fans refer to the different iterations of the MLP toyline. With a lot of their toy franchises, Hasbro will stop producing a toyline once it seems to have run its course, then reboot the franchise for a new generation of kids (and collectors!) to discover. MLP generations are commonly referred to simply as "G1", "G2", "G3", and so on. One common misconception I've seen in recent years is that generations are divided by reboots of the shows/movies; it is actually the toyline, not the shows, that determine the cutoff for each generation.
Gen 1
The first generation of MLP was released in 1982 and ran for a decade, ending in 1992. It was preceded the year before by the short-lived "My Pretty Pony" toyline, but this was the first official release of My Little Pony toys. With hundreds of characters, many different poses and body types, and a lot of international variants, G1 ponies have remained very popular among pony collectors. G1 also had multiple forms of tie-in media, with several animated movies and two cartoon series.
Gen 2
Hasbro's first attempt at relaunching their My Little Pony line was in 1997 with the release of the first G2 toys. These ponies had noticeably different body shapes from the standard look of the G1 ponies. They did not sell as well as expected and were discontinued only two years later in 1999. However, they proved to be more popular in Europe, where they continued to be released until 2003. While this second generation did not have any accompanying movies or shows, it did have a computer game released for PC in 1998.
Gen 3
Hasbro's second attempt at relaunching the franchise proved much more successful. G3 was released from 2003 until 2009 and proved very popular with both kids and collectors. Hasbro once more returned to the chunky bodies and colorful designs that had made G1 popular, but with a more updated look for the new generation. There were several animated specials released alongside this toyline, with VHS tapes and DVDs available with several of the pony playsets.
Gen 3.5
In 2009, Hasbro started scaling back their MLP toyline, choosing to only focus on new releases of seven of their G3 characters -- referred to as the Core 7. Late in 2009, they chose to soft reboot the franchise with the same seven characters in a brand new design and style. These toys were distinctly different from G3, enough to be considered a generation of their own, and were thus named "G3.5" by pony fans. A secondary toyline named "Newborn Cuties" were also released during this time. There were a few animated specials for G3.5 and Newborn Cuties. By the end of the following year, though, these toys were phased out and replaced by G4.
Gen 4
G4 toys started to release in late 2010 alongside the new My Little Pony: Friendship Is Magic cartoon series. For some reason, this cartoon became massively popular with mainstream audiences outside of the pony fandom. These new fans (often referred to as "bronies") were mostly interested in the show, but some did join the collector side of the fandom. This new popularity of ponies lead to a huge increase in the number of pony conventions and fanmade media. Strangely, despite the popularity of the show, Hasbro's G4 line was surprisingly sparse. Most of the toys released only featured the main characters from the show. Even many of the fan-favorite characters only saw blind-bag releases and never received a brushable figure.
Gen 4.5
Similar to the end of G3, the G4 toyline saw a soft reboot in the form of the G4.5 toys released from late 2019 to mid-2021. These toys featured several characters from G4 with the same color schemes, but a smaller and completely redesigned look for the toys. A cartoon series Pony Life was released alongside these new toys. This line wasn't very expansive, featuring only a small handful of toy releases over its short-lived run. It was ended in 2021 with the release of the first G5 toys.
Gen 5
Launching in 2021 with the movie My Little Pony: A New Generation, G5 broke the usual pattern of MLP reboots by instead being a sequel to the previous gen. The canon for the shows (which consists of two separate animated series) is set in the same universe as the G4 main cartoon series. The toyline features several sizes, from larger styling ponies to miniature non-brushable figures. Most of the toys are centered around the main characters from the animated shows.