A backstep is a quick motion backwards, usually almost like a hop; it is implemented in many fighting games. A similar motion in the opposite direction is the frontstep, though other systems may opt to use a dash instead of a frontstep.
The backstep is also known as a “backdash”, particularly in the Capcom world, where a frontstep is sometimes known as a “dash”. In the Samurai Shodown series (where lots of loanwords used in Japanese gaming terminology are often replaced with native-ish equivalents), this is termed “hikikomi” (退き込み).
The first game with a backstep was Fatal Fury, where only Michael Max could use it. The first game where it was usable by players was Art of Fighting (along with a frontstep).
The backstep is essential for regulating range. However, it should be noted that the speed and distance varies according to game and/or character.
Some games give you complete invincibility during a backstep, but even in this case, you are affected by landing stun (see landing cancel).