Fog

Fog (aka Low Hanging Fog, Ground Fog, or Low Lying Fog) is a feature that is seen in various checkpoints. It obscures the player's view, forcing them to blindly throw balls. It can also appear at the ground below the player's field of view.

The fog may be a strategy to prevent the player from seeing unseen features, like ground below eye level, or endless Abyss, or just the checkpoint's stages further ahead. As you get closer, the fog dissipates. Low gravity checkpoints may also use this, to prevent the player from seeing the stage further ahead.

There is no fog in Low quality. Fog is also seen in the "get Premium" menu where it is positioned under a blue door, and the words ingrained in it are "cloud saves".