The sandbox is a new filter that Google applies to nearly all new websites as a way of fighting spammers who figured out the basic elements of Google's ranking algorithm and abusing it. When the the new website is in the "sandbox", it will show in the result pages, but it will not rank well no matter how well optimized it is. Google would still index new websites quickly, but they would they would be on probation until they passed all spam filters. Then they would be allowed to join the rest of the established websites.