This is a great ability to possess for most cards, it adds X to the base power that a card already has, making them stronger and giving the controller of the card less likely to spend more Pillz on the card to power it up.
The best type of Power + ability is called Power=Power Opp., this can be a double-edged sword of sorts. This ability will make the power of the card that possesses this ability to have the base power of its opponent in the current battle. The power will not increase any more passed the base stat. So if your opponent has a Power + ability, you will not gain the power bonus that they would.
Keep this in mind when using cards with this ability.