What does the term Metatile mean ?

In the world of GB, Metatiles are virtual tiles that are larger than 8x8 pixels. In many/most cases, metatiles are 16x16 pixel virtual tiles. In some cases they are 8x16, 16x8, 32x32, or even larger. Metatiles are most often used anywhere that you have large scrolling maps.

Games such as Donkey Kong Land 3, Mortal Kombat 3, Mortal Kombat 4, and Tarzan all use 16x16 metatiles. These games use a metatile map as well. Each position of this map usually contains 1 or 2 bytes. If each position has one byte then up to 256 unique metatiles can be used by the map. These metatile maps can be nearly 4 (or 8 for the GBC) times smaller (ROM space wise) than a similar tile map that doesn't use metatiles. If each position of the map uses 2 bytes then the map can support many unique metatiles. These metatile maps can be 2 (or 4 for the GBC) times smaller (ROM space wise) than a similar tile map that doesn't use metatiles.

Each position of a metatile map has an index into a metatile lookup table. Each position in the metatile lookup table often has 4 bytes (assuming 16x16 metatiles) that index 4 different 8x8 tiles in the tile table and there are often 4 more bytes (assuming 16x16 metatiles) that hold the tile attributes for these tiles for GBC support.

When using metatiles, you lose some ROM space due to the addition of a metatile table but you often gain much more ROM space than you lose due to the much smaller map(s). The space savings often means that you can put a much larger map (pixel-wise) into a single ROM bank which can make things easier programming wise.

The concept of metatiles often assumes as well that 8x8 tiles will be used in multiple metatiles. This is often a very good assumption because the GB & GBC just don't have enough 8x8 tiles available to provide unique tiles at every screen location in large, scrolling maps. It is possible to dynamically load unique tiles at the edges of a scrolling screen (in order to have all unique tiles) but the ROM space required for all of these unique tiles can often be prohibitive.