Since what it is, is pretty well done with some reading from https://www.google.com/search?q=Google+Sandbox&gl=US my question is back to you.
That is, what made you ask? Surely it's not because it's a mystery. What confused you about it?
The Google Sandbox is a metaphorical term to explain why most new websites have very poor rankings in Google Search Engine Results Pages.
The Google sandbox or “sandbox effect” is a filter that Google applies to new websites that want to rank in the search results for specific phrases. The filter exists to prevent SEOs and spammers from easily manipulating search results by satisfying all of Google's major offsite and onsite factors.