Powered (5e Variant Rule)
From D&D Wiki
Weapons with the powered property have a built-in power feature, which usually requires a resource of some kind such as a power cell or battery to function. These weapons are typically futuristic, and would not be found out of place alongside a suit of power armor.
Unless stated otherwise, you use a bonus action when attacking with the weapon to activate its power feature, and the effects of the power feature last until the beginning of your next turn.
In the parenthesis there can be two numbers. The first is the amount of times the power feature can be used before the resource it requires needs to be replenished. The optional second number is shorthand for the damage done by the weapon when using its power feature. Example:
|Chainsword||—||1d6 slashing||4 lb.||Powered (30, 4d6 slashing)|
This chainsword does 1d6 slashing damage normally, but by activating it's power feature it now does 4d6 slashing damage whenever you attack with it until the beginning of your next turn. The power feature can be activated a total of 30 times before you need to replace the chainsword's power cell.
A weapon's power feature, like many other things in D&D, is an abstraction. It not only represents a property intrinsic to the weapon, but also a set of techniques or way of handling the weapon that utilizes it.