Overpower is a Might-class Barbarian skill unlocked at Level 26, which hits all enemies around the Barbarian with a powerful force, dealing damage and providing a variety of buffs courtesy of the rune effects.
This skill feels oddly un-barbaric at first, as it casts like a Wizard Nova attack, and is rather lacking in graphical punch. In fact, it's often hard to tell that it's been cast, with no visual component on the monsters and only a quick pillar of light from the Barbarian up to the heavens for a visual.
While the damage isn't a big deal compare to the Barbarian's variety of attack skills, the effect is as powerful as the visuals are not though, with the rune effects generating substantial bonuses to the Barbarian's various abilities.
Note that a Barbarian with high critical hit chance can reliably reduce the cooldown by quite a bit. The base cooldown is 15 seconds, but when cast in large crowds of enemies, with 8 or 10 or more monsters in range, a Barbarian can reliably knock 4 or 6 seconds off of that time, enabling this skill to be cast quite often, even to the point of keeping rune effects like Killing Spree and Crushing Advance in effect almost full time, at least during big battles with lots of enemies.
The following is a very quick summary. See the Overpower rune effects page for a more thorough description of rune effects, or any of the individual rune pages for numbers, screenshots, blue quotes, and much more.
While the tool tip is very vague, Overpower is a spell-like ability, not a weapon swinging melee attack. It calls down a bolt from the heavens in the base form. The area of effect is over a nearby radius, as can be seen in the video below.
Overpower was first seen in the skill list at the July 2011 Press Event, where it was a level 19 skill. Skill tiers were removed for that event, allowing skills to unlock steadily from levels 1 to 30. Unfortunately, no information about the skill's function was available.
The Diablo III Beta started in September 2011, and while it ran, all of the skills continued to undergo extensive iteration. Skill changes varied from changing the levels the skills are unlocked, to changes in resource cost, damage, and even function. A few skills were removed altogether, and a few new ones were added. Also, the entire skill system received an overhaul with the Beta Patch 13 update in February 2012, with physical runes replaced by rune effects that automatically unlock at various levels for the different skills.
Overpower was moved to level 18 for the beta. With the Patch 13 changes, it was classified as a Might skill (but stayed at level 18). For the Beta Patch 14, it was bumped up to level 26.