The Sandbox is a name of an observation that how the Google ranks web pages in its index.
The Google Sandbox Effect
It is the restriction placed on new websites which is an alleged. As a result of which, a new website does not receive good/higher rankings for its most important keyword phrases and keywords for few months. A new website may still affected adversely by the Sandbox effect even with many inbound links and good content.
For new sites, the Google Sandbox acts as a probation to discourage spam sites from getting banned, rising quickly etc. Therefore, the Google Sandbox is too much similar to a new site being placed on probation whose rank is kept lower than expected, prior to give value for its content and inbound links.
Google reduces the page rank of new domains temporarily and place them into its “sandbox” to counter all the ways that to manipulate Google’s page ranking, search engine optimizers attempt by creating many inbound links to a new site from other web sites that they own
A “reverse sandbox” effect can also exist where new pages with good content but without inbound links are increased in rank temporarily.
How long a site is stays in the Google Sandbox?
From one to six months, stays of website in the Google Sandbox can vary and with three to four months being the average time. In the sandbox, less competitive searches will be given very shorter stay. Then the filter will be decreased gradually over time and will lose its dampening effect (most of it) in about three months. Whereas, the Sandbox filter might remain for six months for the most competitive search keyword phrases.
So, I hope you now understand very well about this Google Sandbox.