CPC stands for Cost Per Click.
This metric in Ahrefs shows the estimated bid (in US dollars) for a given keyword in Google Adwords.
CPC is a very dynamic metric, because advertisers can raise or lower their bids anytime. Therefore, the number that you see in Ahrefs should be treated as an average, because we don’t update our CPC data very often.
If you need information about how CPC is calculated by Adwords, check out this article: https://support.google.com/adwords/answer/6297?hl=en