Avengers: Endgame spoilers ahead. At the end of Avengers: Infinity War, Thanos snaps his gauntlet-covered fingers and erases half of all the living creatures in the universe. Then, he goes and sits on the porch of some sort of little cottage and gazes out at the landscape, knowing things are now "balanced." But, where is Thanos after the snap? Because at the start of Endgame, he's still there and it's where the Avengers find and eventually kill him.
Early on in Endgame, when the remaining Avengers are looking to track down Thanos, retrieve the Infinity Stones, and reverse everything, Nebula reveals that Thanos often told her what he would do after achieving his goal, which Rhodey refers to as Thanos's "retirement plan." Along with this information, the Avengers are also able to locate a power surge from when Thanos last used the Infinity Stones that leads them to him. Very quickly — suspiciously quickly — the Avengers reach the planet where Thanos is living as a minimalist farmer, but unfortunately, he's already used the stones to destroy the stones. Then, Thor suddenly and easily kills Thanos, and the movie really takes off from there with the Avengers feeling like they're all out of options.
As for where exactly Thanos is during all of this, in the movie it's referred to as "the garden" and it's officially called Planet 0259-S, according to Vulture and this incredibly short entry on the MCU Fandom page. So, it's not Thanos's home planet of Titan, which Infinity War and Endgame directors Joe and Anthony Russo explained during a Q&A hosted by Collider in December 2018. Screen Rant reported that the directors had been referring to the planet where Thanos goes after the snap as "Titan 2", although that isn't a name used in the new movie.
While there isn't a lot to know about this planet specifically from the movie, Thanos living as a farmer does have a comic book history. As noted by Den of Geek, at the end of the 1991 Infinity Gauntlet comic miniseries, Thanos fakes his own death and moves to a planet to live alone as a farmer. While Infinity War and Endgame don't stick to the plot of the comics, they do pull aspects from them. In addition to Thanos retreating to a "backwater planet", as an article on Polygon calls it, the Infinity War movie ends where the Infinity Gauntlet series begins, with Thanos having collected all of the Infinity Stones. As you can see, the stories are very much connected, but they're not even close to being the same.
There is also a character in the comics called the Gardener (aka Ord Zyonz), who was introduced in 1977. Notably, he is featured in The Thanos Quest, which came before The Infinity Gauntlet. In that comic, Thanos gets the Time Stone from the Gardener, who is one of the oldest beings in the universe. The Marvel Database describes the Gardener as follows:
"Virtually immortal, the Gardener devoted his life to the creation of natural beauty by sowing the seeds of plants, flowers, and trees upon barren or devastated worlds. The number of worlds he has turned into fertile planet-wide gardens over the eons is countless."
While this character is not mentioned in Endgame, perhaps he provided some inspiration for the very fertile world Thanos ends up on.
As for what this all means — Ord Zyonz involvement or not — is basically just that Thanos has achieved his life's work of "restoring balance," so he doesn't have anything else to do except remove himself from the half of all living creatures who are still alive and hate him.
Infinity War and Endgame screenwriters Christopher Markus and Stephen McFeely spoke to the New York Times about the film and explained that killing Thanos on that planet just ended up making sense considering his motivations. "It reinforced Thanos’s agenda," Markus said. "He was done. Not to make him too Christ-like, but it was like, 'If I’ve got to die, I can die now.'"
That's pretty clear, so maybe "the garden" wasn't mean to serve as a metaphor beyond that. But, a line stands out from a post-Infinity War article on The Wrap from April 2018 about Thanos' reason for the snap: "Basically, Thanos considers himself a cosmic gardner, and he needs to prune everything back a little so the whole garden doesn’t get overgrown and run out of resources."
Once a gardener always a gardener? Well, until Thor chops your head off, at least.