google it...its a box with sand ... google keeps new sites or sites that are trying to beat the system in the sand for a period of time so they do not get an unfair advantage over sites that have been out for a period of time. The Sandbox (a.k.a. Sandboxing or the sandbox effect or the Google penalty) is a name given to an observation about the way Google ranks web pages in its index. It is the subject of much debate—its existence has been written about[1][2] but not confirmed and several observers state that they have observed the contrary.[3] According to the sandbox effect, Google temporarily reduces the page rank of new domains, placing them into what is referred to as its "sandbox", in an effort to counter the ways that search engine optimizers attempt to manipulate Google's page ranking by creating lots of inbound links to a new web site from other web sites that they own. Some SEO experts also claim that the sandbox only applies to highly competitive or broad keyword phrases, and can be counteracted by targeting narrow, or so-called long-tail phrases.[4] A "reverse sandbox" effect is also claimed to exist, whereby new pages with good content, but without inbound links, are temporarily increased in rank—much like the "New Releases" in a book store are displayed more prominently—to encourage organic building of the World Wide Web.[3][5] David George disputes the claim that Google applies sandboxing to all new web sites, saying that the claim "doesn't seem to be borne out by experience". He states that he created a new web site in October 2004 and had it ranked in the top 20 Google results for a target keyword within one month. He avers that "[n]o one knows for sure if the Google sandbox exists", and comments that it "seems to fit the observations and experiments of many search engine optimizers". He theorizes that the sandbox "has introduced some hysteresis into the system in order to restore a bit of sanity to Google's results".[3]
In short, 'Google sandbox is a hypothetical filter that restricts newer sites from ranking well for competitive and high volume keywords'.
Sandox is a form of penalty of Google for too quick increase of external links, which lead to WWW page.
It is google filter which keep prevents new sites from top ranking. It can take up to 12 months to get out of sandbox. Building links and adding fresh content on regular basis might reduce the time your site is in sandbox.