Nametable
The NES background is a grid of tile numbers stored here, each byte pointing into the pattern table for the actual pixels.
The NES Picture Processing Unit builds its background from a 32×30 grid of tiles. Each cell is one byte indexing a tile in the pattern table, with colour supplied separately by the attribute table. The console's VRAM held two physical 2 KB tables; cartridge wiring mirrored them into four so the picture could scroll in any direction.
Crossing a tile boundary while scrolling forced the game to write fresh tiles into the far table during vertical blank — the only window when the PPU wasn't reading memory. Super Mario Bros. (1985) drove its side-scrolling exactly this way.