Google SandBox effect is considered to be form of treatment that new sites get once they are online or after a period of 3 months being online. Ann Smarty at SEJ gives a good synopsis of what it is.

An excerpt from SEJ

1. "Google Sandbox" is supposed to mean a period of time before a newly launched website is properly indexed and ranked. They say, Google deliberately holds back indexing new sites before they "earn its trust".
2. "Google Sandbox" can also mean a sudden drop of SERPs that happens about 3 months after a site was launched.

The system seems to be a way for Google to monitor new content that enters its index. The age of a site is supposed to be in important factor in its credibility ( and also the outbound and inbound links growth rate). This does not however mean that a site is not indexed at all but only means that the site will not appear for competitive keywords.