In computer security, a sandbox is a security mechanism for separating running programs. It is often used to execute untested code, or untrusted programs from unverified third-parties, suppliers and untrusted users. Or Sand box is a time period in which your site gets back links.
new sites sees good rank initially, then for a few months sees lower rankings. some people call that sandbox.
Untill the year 2009, Google has applied a sandbox to most of new sites. A website, which got into sandbox could be only found by a very specific keyword combinations from the content of the website. Therefore, such websites normally got 0 visitors from Google. From March-June 2009, Google has basically eliminated that sandbox due to his infrastructure empowerment. However, some artefacts remain, like in the post from webcosmo.