Power is the strength of which your card makes its attacks. It's written there on the card, eg. Dorian has a power of 8 and a damage of 7. And attack is the total of your cards power multiplied by the number of pills plus the original power.
Example : Power = x, Pills =y and we'll use Dorian.
So x = 8 (Dorian's power) and lets say we use 4 pills, so y=4, thus, you have :
(8*4) 8 = 40, so the attack is 40.
Hope that is clear for you.