Unreal has 2 main Threads, the GameThread and the RenderThread. GameThread is for all the logic stuff like Game-Loop, Tick, Replication (Multiplayer), Timers, …
The RenderThread does all the rendering. To not get into any thread problems cause of raceconditions and for performance reasons (time of access to the data), each object that is to be rendered (mainly UPrimitiveComponent) provides a proxy object (SceneProxy) that the RenderThread can safely access. Any object state that influences rendering - like transform changes - need to get updated to the proxy object (This is called MarkDirty most of the time, see UInstancedStaticMeshComponent::UpdateInstanceTransform for example).