Parallax

Parallax is an option which controls how much the underground backgrounds scroll relative to foreground objects such as walls, blocks, and NPCs.

Settings from 0 – 100 (inclusive) are available;
 * A setting of 0, the default setting, will make the background "stationary", causing it to follow player's movement, just as foreground objects do.
 * A setting of 100 will make the background scroll with the player's movement at a relatively high speed.