What is Google Sandbox?

Google Sandbox is some kind of of Google penalty, which filters out backlinks to a penalized site which would otherwise have a positive impact on the site’s rankings in Google.
The Google Sandbox Effect is a theory used to explain why newly-registered domains or domains with frequent ownership changes rank poorly in Google Search Engine Results Pages (SERPS). In other words new websites are put into a “sandbox” or a holding area and have their search ratings on hold until they can prove worthy of ranking.
Google Sandbox Theory is that Google puts new websites into a “Sandbox” before they can rank well in a Google search and make their way to the top of a search result. Google wants to make sure that a website is legitimate and not just a fly-by-night website that was set up with bad intentions before it ranks it prominently on the SERP (search engines results page).