Sidestepping (also known as "sidestep dodging", "spot dodging", "ground dodging", or simply "dodging") is a maneuver that renders the character invincible for a short period of time. It is performed by pressing down (on the Control Stick or the C-stick) while holding a shield button (L, R or Z). Its main use is to avoid attacks.
Sidestepping is the fastest and shortest out of the three kinds of dodges (the others being the rolling dodge and the air dodge).In Melee, it's also the only one out of the three that does not move the character.
While the sidestep animation involves the character leaning into the background or foreground, the invincibility frames gained dodge all attacks, including those that come from the foreground or background.