Any move that dispatches an attack hitbox covering the entire screen is a full-screen attack. It will affect the opponent in some way if allowed to start up.
Most full-screen attacks have projectile hitboxes. For this reason, you can rarely expect one to ever trade hits, and if it has startup, invincibility and/or other effects prepared, it is powerful.
Depending on its active timeframe, there is often no way to evade it, and no choice but to block once it activates. In addition to that, it is usually a super special move, with chip damage taken to the extreme. For that reason, it can often be used to guarantee a K.O. (see chip K.O.) when the opponent’s life is down low. This is, admittedly, pretty cheap.
The first full-screen attack was probably from X-Men: Children of the Atom, and as in that game, full-screen attacks are usually only found either on last boss kind of characters, or useful as a flashy move only.