Draw call
A single command telling the GPU to render a batch of geometry — issue too many and the CPU, not the graphics card, becomes the bottleneck.
Each call carries a fixed cost: the driver validates state and prepares the card whether the batch is three triangles or three million. A frame budget typically allows a few thousand calls before the CPU can no longer keep up.
You cut the count with batching, instancing, texture atlases and merged meshes. "Draw-call bound" means CPU-limited. The leaner Vulkan and DirectX 12 APIs (2015–2016) slashed the per-call cost compared with the older OpenGL and DirectX 11, precisely to relieve this bottleneck.