When your fellow users vote up your questions and answers on a Stack Exchange site, you generate reputation. Reputation is a rough measure of:
- how much the community trusts you
- your communication skills
- the quality and relevancy of your questions and answers
These friendly reputation leagues are an informal way of tracking your reputation within the community on a particular Stack Exchange.
Reputation is capped at 200 per day, but remember that bounty awards and accepted answers are immune to this daily reputation cap.