Credits are used to redeem x10Rewards (check the panel to the right) or buy things from the vBCommerce/Market subforum.
You can get points by posting, making threads, selling services, getting donations, winning raffles/promo contests or referring people (check Referral Center to the right).
Note: Referral system may not work/work fully... I asked a friend to click my link and just come to the site, but the referral center doesn't say I have any clicks...however I've gained 2 unexplained credits... I had him try twice because he had some plugin blocking scripts... so maybe that's 1 point each link follow or 2 for the first one?