The Google Sandbox means that has been coined to describe the effect that new website have in Google’s Search results.
New Websites do not behave in the same way as older, more established websites in Google’s Search Results.
It seems that Google is less inclined to rank newer web portal until they have proved themselves and gone through a ” probationary” period. This seems to be a period of between 90 and 120 days.
The Google Sandbox could be used by Google effort many things other than just new website. Google could also use this for websites that it believes are not playing fair or that have suddenly gained large volumes of inward links.
If you have a new website, you should make your changes and optimize it gradually. This includes building links to your website.