An LED wall is thousands of modules pretending to be one screen, and the processor maintains the pretense: it takes HDMI, SDI, or network video in, scales and slices it, and distributes the pieces to panels with matched color and timing. Brands like NovaStar and Brompton own the racks beside most walls.
For performers it is the machine your laptop actually talks to. It advertises an expected input resolution, adds a frame or so of latency, and quietly rescales anything nonconforming, softening the image.
The soundcheck ritual: ask the wall tech for native input specs, output exactly that, and confirm a test pattern edge to edge before doors.