Minecraft's 1.17 update launched a few weeks ago, but that was only part one of what's collectively known as the Caves and Cliffs update. We've known what's coming in 1.18 for a while, and the key feature, new terrain generation, is now available to play via an experimental beta.
A post on the Minecraft site explains how to get onboard. There are technical reasons why the snapshot can't be rolled into the regular Java beta, and so this is being flagged as more explicitly experimental. That aside, the changes are exciting enough to make it tempting.
Basically, 1.18 completely remakes how Minecraft generates its world. There are new mountain biomes that offer higher peaks, new cave biomes that should make underground exploration feel fresh, and a new ore distribution that's going to mess with all your old routines about where to find diamonds. They've provided a useful visual chart for the new ore distribution, at least.
Since this is such a major change, 1.18's beta won't be able to open worlds generated in earlier versions of the game. You'll need to start a new world fresh.
The same update is also available on Bedrock, which gets its own, similar post about the update and how to access it via version 1.17.10.
If you want to know more about Minecraft's recent updates, you should check out Ollie's near-infinite guides. For example, yesterday he explained how to get bees in Minecraft.