Static batching
Merging many non-moving meshes that share a material into one mesh, so the GPU draws them in a single call.
Every draw call carries CPU overhead — state changes, driver validation — so a scene of a thousand separate rocks can go CPU-bound long before the GPU breaks a sweat. For geometry that never moves, an engine can pre-combine everything using a given material into one large vertex buffer at build or load time, collapsing a thousand calls into one. The cost is memory: the combined mesh is stored on top of the originals, and the whole batch redraws even if only a corner is visible, so it interacts awkwardly with culling. Unity popularised the static/dynamic split; the underlying idea is as old as the draw call itself.