The ending of Ice Age Buck Wild is simple on the surface: the fire-breathing lizards turn Orson's raptors against him, his attempt to take over the Lost World collapses, and Crash and Eddie choose to stay there. Manny, Sid, Diego and Ellie promise to visit often.
What does it mean? In my opinion, the clearest reading is that the story is about growing up rather than about beating a villain. The brothers leave camp because the herd doubts them, and the film then answers that doubt scene by scene. Buck is captured and the raptors hold the watering hole, so the herd has to follow Crash and Eddie into a fight the brothers understand better than anyone. They team up with Zee and the Tyrannosaurus called Momma, and the lizard trick is theirs, not Manny's. Winning an argument by saving a whole world makes staying a choice rather than a punishment. It could be argued that the herd's promise to visit often is the film's way of showing that the trust has finally been earned.
Orson matters here too. Buck explains that the small Protoceratops was once banished for his ambitions, so he is a mirror for the brothers: another young animal who was told he was not ready. The difference is that Crash and Eddie use their wild ideas to protect others, while Orson uses a pack of raptors to rule. That being said, I think the film is lighter on his motives than the villains of the theatrical run, and he works mostly as a threat that lets the brothers shine.
The herd's role also stands out when you compare it with earlier films. In Ice Age: Continental Drift (2012), Manny is the one vowing to get back to his family across the sea, and in Ice Age: Dawn of the Dinosaurs (2009) the herd mounts a rescue after a huge Tyrannosaurus mother carries Sid off to a hidden world under the ice. In both, the old guard leads. Here the herd joins a fight already under way, and Manny, Sid, Diego and Ellie end up helping the brothers rather than rescuing them from their own mistakes. In my view that handover is the most meaningful change in the film.
The ending also reads as a setup. According to TheWrap, the film began as a Buck Wild TV spinoff and was reworked into a feature after Disney acquired 20th Century Fox, and it works as a pilot for the Buck Wild series. A new home for Crash and Eddie in the Lost World gives a series an obvious base, with Buck as the guide. The first five films were made by Fox and Blue Sky Animation Studios, and none of them ended by moving two main characters out of the herd for good. Even so, the film plays fine as a one off, and the farewell lands without any promise of what comes next.