YMT is the metadata sibling of YMAP. If YMAP files tell the game 'this object goes here', YMTs tell the game about the properties of those objects - their bounds, LOD behaviour, variants.
What YMTs typically contain
Collision bounds. Texture variants. Sound tags. LOD distances. Spawn-region filters. They're small files, but they pack a lot of information into them. Files you forget about until something doesn't load properly, and the YMT is to blame.
Most modders only interact with YMTs when adding new props that require engine-side rules. Modifying existing YMTs is risky, because one bad edit can break loading.