There are several posts about the reputation process here. If you type "reputation" into the search bar, I'm sure many of them would pop up for you.
Basically, members who have higher rankings (such as knowledgable or adept) are able to rank other members here on the site. Reputations come on varying levels: fluffy (which only high ranking members can see), beginner, novice, knowledgable and adept. Rankings are given to members who participate in the chatter and forums, prove what they know, show maturity and experience or knowledge of various things, etc.