Resource Credits are new. They are the equivalent of bandwidth on the old system. What you need to know is that you only have a finite number of "moves" per day on the blockchain. The more steempower you are holding, the more moves you can make. When offering free transactions, like the steem blockchain does, where resources are not infinite. Something does have to give. That is where resource credits some in.
So, if you go to steemd.com/@yourusername you can see exactly how much RC's you have on your account.
Here is a shot of mine at the moment... it seems like I have a clean slate at the moment ;)
Use steemd.com as a tool too see where your voting power and resource credits stand, and to see some of your most recent transactions etc.
I hope this helps.