The Google Sandbox is very similar to a new website being placed on probation, and kept lower than expected in searches, prior to being given full value for its incoming links and content.
The result is that a site does not receive good rankings for its most important keywords and keyword phrases. Even with good content, abundant incoming links and strong Google PageRank, a site is still adversely affected by the Sandbox effect. The Sandbox acts as a probation for sites, possibly to discourage spam sites from rising quickly, getting banned, and repeating the process.