Google Sandbox Definition
“Google Sandbox” is the description of the effect of a Google Search Engine Algorithm which is used to prevent new websites from ranking too high in the SERPS (Search Engine Result Pages), mainly to filter out spam pages.
Google Sandbox effect - results
The “Google Sandbox effect” excludes inbound links to new websites from the calculation of the SERPS rankings for a certain period of time, typically 3-6 months. During this time, a new site is usually ranking worse than it would without the Sandbox effect. As soon as the Sandbox effect is over, the site’s ranking is calculated normally and the SERPS rankings are abruptly improved.
Google Sandbox - how does it look like?
There is no real box created by Google that is filled with sand, “Google Sandbox” is just metaphorical speech.
However, if it existed, it might look like this:

[…] damping effect is also called Google Sandbox. Having a high Google trust factor enables a website to avoid the damping that Google applies to […]
[…] links from parasite host sites, it is possible to leverage Google Spam Filters, such as the Google Sandbox Effect and a certain filter applied to link popularity which prevent new websites from achieving a good […]