A YMAP file is what GTA V uses for its map data - where objects are located, at what rotations and with which variants. If you got here from elsewhere on the site, that's enough info for now.
Intentionally left short. An anchor for the glossary.
What's actually inside a YMAP
Object placements. Each entry references a model (generally a YDR file), defines a position in world space, defines a rotation (as a quaternion), and optionally specifies LOD and lighting parameters. A YMAP essentially contains the level-design information - what objects are where, in what orientation, at what scale.
Some YMAPs are enormous - an entire city block. Some are minuscule - adjusting one prop. The format is XML-inspired by lineage, tightly packed binary in practice.
How YMAPs are edited
CodeWalker is the standard tool. You open a YMAP file, see objects rendered in 3D, move them around, add new objects, save your file. You can also edit the XML itself by hand, but it's painful.
Adding your own custom interior in GTA V, removing an existing prop or moving a building - it's all considered YMAP work. YMAP is the format used for the working layer of map modification.
Why this matters for GTA 6
GTA 6 will most likely use a successor format. Whether it's a recognisable iteration of YMAP or an entirely different file structure, nobody will know until GTA 6's PC release. Then the community will reverse-engineer whatever it ships with, and a CodeWalker successor will follow.