A YMAP file is the file format GTA V uses to define map data - what objects exist where, with what rotations, with what variants. If you have come to this page from another article, that's the short version.
We keep this short on purpose. Glossary anchor.
What's actually inside a YMAP
Object placements. Each entry references a model (a YDR file usually), gives a position in world coordinates, gives a rotation as a quaternion, optionally specifies LOD and lighting parameters. A YMAP is essentially a level-design file - what's where, in what orientation, at what scale.
Some YMAPs are gigantic - a dense city block. Some are small - a single prop adjustment. The format is XML-derived in spirit, packed binary in practice.
How YMAPs are edited
CodeWalker is the standard tool. You open a YMAP, view the objects in 3D, move them, add new ones, save the file back. Hand-editing the underlying XML is possible but painful.
Adding a custom interior to GTA V - or removing an existing prop, or shifting a building - is YMAP work. The format is the working layer of map modification.
Why this matters for GTA 6
GTA 6 will probably use a successor format. Whether it's a recognisable evolution of YMAP or a substantially different file structure is unknown until the PC release. The community will reverse-engineer whatever ships and a CodeWalker successor will follow.