A MESHLIB file contains a mesh library created by Godot Engine, a free and open source game engine used to create 2D and 3D games. It stores information about one or more mesh resources, which includes vertices, edges, and faces, used to describe the shape of 3D objects in the game. MESHLIB files allow users to drag and drop meshes from the library in Godot Engine instead of constantly having to import the meshes one by one.