I'll, try explaining in simple words.

Voting Mana = Voting Power

RC also known as Resource credit =  allows you to perform on the steem blockchain

Voting power is associated to the Steem you have vested. So, higher the number of steem you have vest, higher is your vote influence through the Voting power.

RC in other terms refer to the bandwidth or resource you use on the steem blockchain. RC, is directly associated to the steem you have vested i.e. higher the amount of steem you have vested the higher the RC you have and  vice versa.