It refers to a commonly held belief that Google has a filter that places all new websites under restrictions for a certain amount of time to prevent them from ranking in searches.

It is an imaginary area where new and less authoriative sites are kept for specific time period. until they established themselves of being displayed on the search results.

The google puts a new website in the sandbox before rank well in google search make the way to top of the search result google want to make sure their website is legitimate it is act as filter in the search engine it is "imaginary part".

It is an imaginary area where new n less authority sites are kept for specific time period

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.

why do not you google search before asking questions here?

Google Sandbox is generally used for websites which are newly launched and then arouse the suspicion of Google, usually by adding a large amount of new content in a very short period of time. This appears to Google as spam; and the Sandbox is essentially a spam filter which watches for suspicious activity, reasoning that a site which adds enormous numbers of new posts, pages and content instantly is probably filling itself with duplicate content or engaging in search engine spamming.