The Zero Laser is Samus' Final Smash. Samus charges her arm cannon and fires a large beam of energy that deals a high amount of damage. After firing the beam, Samus' Power Suit is "damaged" and falls off of Samus' body, transforming her into Zero Suit Samus.
After the beam has been fired and the Power Suit removed, the damaged pieces of the suit can be picked up and thrown as items by any player.
By holding a specific button, Samus will transform into Zero Suit Samus. The transformation animation occurs during the 3 second countdown and works the same way as if the Zero Laser had just been fired beforehand.
In a posted video, it shows Samus using her Final Smash. It lasts about a total of 4 seconds. The move's damage varies from source, but depending on the time the opponent remains in the laser, it can do from 40-60% damage.