I’ll give you a really simplified explanation. If it doesn’t satisfy your question, you probably don’t need the Priority service very much. You can learn more details from google blog though, if you’re interested.
Many people receive far more email than they can possibly read. The longer you have an email account, the more spam it will receive (Nobel Prize hopefuls are trying to formulate the exact relationship). Plus, there are plenty of non-spam emails that just don’t deserve your full attention (think of blast emails, chain emails, forwards, anything where your name is one of hundreds on a cc or bcc list). Then there are the notification emails from banks, credit card companies, etc and then there are the “sales” announcements from every retailer whose site you’ve ever visited. Once or twice a week there is that one email from your mom that if you don’t notice immediately and reply to she will be reporting you as a missing person.
If you are like many people, you need a secretary to sort your email into piles just to get down to the one or two that you actually care about. Priority Inbox tries to do that for you.
(c) 2010