Like its preceeding test versions, this snapshot ("1.18 experimental snapshot 4") makes changes to existing world generation, in this case making changes to the placement of various terrain features in a world.
In this snapshot, badland and desert biomes are now larger and extreme hills are now even more extreme.
Biomes in general have had their positioning in the world tweaked to avoid harsh boundaries between different biomes, and now generate less noisily, reducing the likelihood of tiny biomes being placed into a world.
Changes have also been made to structure placement. Jungle and desert pyramids will now no longer generate floating mysteriously on bodies of water.
Finally, river placement is now more natural, with rivers now creating fjords in mountain ranges instead of being weirdly placed.
The full release of 1.18, Caves & Cliffs Part II, is set to be at the end of this year. Proper snapshots for 1.18 are expected to start in the coming weeks.