A .umap is an Unreal Engine level. Historically it contained everything placed in that level: every actor, its transform, its property overrides, and the level's own settings, all in one binary file.
That design made the level file the classic collaboration bottleneck in Unreal team development. Because it is binary and unmergeable, only one person could meaningfully work in a level at a time. Teams worked around it with sublevels, carving the world into pieces assigned to individuals, which worked but meant the level's structure was driven by team headcount rather than by the game.
Unreal Engine 5 changed this. With World Partition and One File Per Actor, actor data moves into external per-actor files, and the .umap shrinks to a shell holding the streaming grid configuration and data layer definitions. Several people can now work in one world simultaneously.
The .umap still matters, and it is still binary. Changing partition settings or creating a data layer touches it, and two people doing that at once still conflict. It went from being the bottleneck for everything to being a shared file you touch occasionally and should lock when you do.