You can manually combine multiple meshes into a single mesh as a draw call optimization technique. Unity renders the combined mesh in a single draw call instead of one draw call per mesh. This technique can be a good alternative to draw call batching in cases where the meshes are close together and don’t move relative to one another. For example, for a static cupboard with lots of drawers, it makes sense to combine everything into a single mesh.
Warning: Unity can’t individually cull meshes you combine. This means that if one part of a combined mesh is onscreen, Unity draws the entire combined mesh. If the meshes are static and you want Unity to individually cull them, use static batchingA technique Unity uses to draw GameObjects on the screen that combines static (non-moving) GameObjects into big Meshes, and renders them in a faster way. More info
See in Glossary instead.
There are two main ways to combine meshes: