The Charge Shot is Samus's Neutral B move.
Pressing the special move button will cause Samus to start charging energy in her arm cannon, which can be fired by pressing the button again. When the attack has reached maximum strength, Samus will stop charging and resume her neutral standing position, but her arm cannon will start to flash. Pressing the special move button in this state will let her fire the fully charged shot. Alternatively, the player can press grab, shield, or the control stick left or right to cancel the charge and/or dodge, and pressing the special move button again will resume her charge. If Samus is hit while she's charging, she will not retain the stored charge.
When used in the air, Samus will automatically fire either a non-charged blast, or however big a shot was charged previously. A fully charged shot does 26% damage. The attack does great knockback at higher percents and is a deadly projectile for edgeguarding. A fully charged shot can also break a Smash Ball in one hit (though this only happens 5 seconds after the Smash Ball appears).
Origin
If she didn't start out with it, the Charge Beam (as it is called outside of Smash Bros.) was one of the first power-ups Samus would get in the Metroid games, starting with Super Metroid. A fully charged shot would become Samus' only way of damaging certain bosses should she run out of Missiles. In Metroid Prime, Charge Shots can be used to clear debris blocking Samus' path or to suck in nearby health and ammo, as well as a starting point for Charge Combos such as the Super Missile. In none of these situations can Samus save a charge for later as seen in Smash Bros.
Charge Cancel
In Brawl, Samus is capable of interrupting the charge of the attack by pressing the shield button. Since it's possible to go directly into a jump while shielding, Charge Canceling can be used to fake out an opponent by allowing them to come close to you while you're charging, only to find themselves attacked by an aerial. Because the shield button was held down to cancel the shot, many players that are just learning the basics of this technique can only perform an aerial Grapple Beam afterwards, since they do not let go of the shield button quickly enough. However, with fast timing, it becomes possible to perform any action that is possible at any other given time that Samus would be airborne.
As an added bonus, the amount of time the Charge Shot had been charged is still retained after canceling via shield, just like if the player were to cancel it via a roll.
Gallery
Samus's arm cannon flashes at the end, a sign that her power beam is fully charged
Samus charging the power beam in Melee
Samus shooting a fully charged power beam in Melee
