Google sandbox in essence is a holding area for new domains or domains that are seen to change in ownership frequently. Some people have claimed it could take up to a year for a domain to be released from the sandbox, we have not experienced such time lengths but have witnessed occurrences of three months or more.
It is said to help cut down new SPAM domains or understand the value of a new domain before it is ranked and allowed to rank for the desired search term phrases.
See FAQ
getting out of the Google sandbox to minimise this happening to your new domain.