What Do You Mean by Google Sandbox in SEO?

[AshokDixit89]

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.

Effect of sandbox
When your site is put into the Google Sandbox, your site is still indexed and crawled, but you will be set aside when it comes to actual Google searches. Until you make it out of the Sandbox, you won't show up in Google's search results.