Render thread
A dedicated thread that feeds the GPU the previous frame's draw calls while the game thread is already simulating the next one.
When multi-core consoles like the Xbox 360 and PS3 arrived in the mid-2000s, running the whole game loop on one core stopped being good enough. The standard split became a game thread for logic, physics and AI, and a render thread that translates the result into command buffers. They run staggered: while frame N is being drawn, N+1 is being computed. Unreal adds a third, the RHI thread, sitting closest to the graphics API.
Two threads can't touch the same data, so the engine keeps a mirrored copy of the scene for rendering — Unreal calls these proxies. The price is an extra frame of latency, which shows up as input lag and is why twitchy shooters sometimes choose to turn the pipelining off.