It is the number of inquiries you can get. These credits refresh every month depending on your rating. As long as you have enough available, you stay visible in search and can continue receiving inquiries from different clients.
How much inquiry credits do I get? Depends on your rating. The better your reviews, the more credits you get! To learn more: http://raketph.zendesk.com/hc/en-us/articles/206898447-Becoming-a-Rockstar-Raketeer
Does this apply to Clients? Yes - Clients require a Premium Subscription to be able to send inquiries to Raketeers.