A nametable is a 1024 byte area of memory used by the PPU to lay out backgrounds. Each byte in the nametable controls one 8x8 pixel character cell, and each nametable has 30 rows of 32 tiles each, for 960 ($3C0) bytes; the rest is used by each nametable's attribute table. With each tile being 8x8 pixels, this makes a total of 256x240 pixels in one map, the same size as one full screen.
(0,0) (256,0) (511,0) +-----------+-----------+ | | | | | | | $2000 | $2400 | | | | | | | (0,240)+-----------+-----------+(511,240) | | | | | | | $2800 | $2C00 | | | | | | | +-----------+-----------+ (0,479) (256,479) (511,479)
The NES has four nametables, arranged in a 2x2 pattern. Each occupies a 1 KiB chunk of PPU address space, starting at $2000 at the top left, $2400 at the top right, $2800 at the bottom left, and $2C00 at the bottom right.
But the NES system board itself has only 2 KiB of VRAM (called CIRAM, stored in a separate SRAM chip), enough for two nametables; hardware on the cartridge controls address bit 10 of CIRAM to map one nametable on top of another.
Conceptually, the PPU does this 33 times for each scanline:
It also does a fetch of a 34th (nametable, attribute, pattern) tuple that is never used, but some mappers rely on this fetch for timing purposes.