The actual cost-per-click (actual CPC) is the final amount you are charged for a click on your advert. You are often charged less, sometimes much less, than your maximum cost-per-click (max. CPC) bid. In other words, actual CPC is the absolute minimum an advertiser needs to spend in order to maintain their ad.
Ex. “I’m willing to bid as high as $1.75 for my wedding photos ad, but luckily my actual CPC is only $1.60.”