This is a quote from the thread explaining the BioCredits system:
Reputation has a massive effect on your BioCredit total. If you make helpful posts, other members will give you a +1 bonus to your reputation, which will count towards 10 BioCredits. Anyone who has a post count of 1 or greater can add or remove reputation from other members, and the amount of reputation that you can give/remove increases directly with your own reputation score.
Basically, it's one way of judging how helpful (or otherwise) other members think you are.
One other point to note: once you have given someone some reputation points, you won't be able to give the same person any more until you have given some to 3 other people.