Google Sandbox is an imaginary area which contains new and less authoritative sites for a specified period until they cannot be displayed in the search results. It is an alleged filter for the new websites. In simple words, we can say that it places new websites on probation and ranks them lower than expected in searches. It may be caused by building too many links within a short period of time.
Any type of website can be placed in a sandbox. However, the new websites who want to rank for highly competitive keyword phrases are more prone to the sandbox. There is no fixed duration for a site to stay in the sandbox. Generally, a website can stay in the Sandbox for one to six months. The logic behind the sandbox is that new websites may be not as relevant as older sites.