Low altitude refers to the height of something that is in the air, but not very high. Depending on the game’s system and the context of what is being discussed, this can refer to the height of small jumps and backsteps, as well as the beginning and end of jumps in general.
Indicating that something is at low altitude or interacts with low altitude actions is important, because the (lack of) height means less vulnerability in general, while still retaining some of the advantages of having the height at all.
For example, low altitude attacks can be used in mixup (assuming the jump attack or special move you use has an overhead hitbox), zoning (using the lack of height to reduce how long you are open), and certain combos.
Certain tricks using the buffer can be useful from time to time in order to use a midair special move very close to the ground.