The Google Sandbox is an alleged filter placed on new websites.

The Google Sandbox is an alleged filter placed on new websites. 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.

These is an imaginary area where new website are put on hold until they prove they worth of the ranking this will be act as an filter for the search engine.