Volt Tackle is Pikachu's Final Smash attack in SSBB. When Pikachu uses this attack, he temporarily turns into a gigantic ball of light. The ball of light will then fly around the stage at great speeds, and the direction Pikachu goes during this move can be controlled by the player.
Volt Tackle is named after a secret move in the Pokémon Advance games that only members of the Pikachu family can learn. This move in Pokémon is a direct reference to the signature move of the titular character of a Genesis game called Pulseman made by Game Freak. The move's look and controls in Brawl are also direct references to the move's look and controls from Pulseman.
External Links
[1] The Volt Tackle can be seen near the end of the clip.