there is an entire section just about the sandbox, you could just read a bit and find out by your own.. or if you want, read this website: http://www.14thc.com/google-sandbox.html I found it interresting, I'm not sure if every info on it is valid, but it sure does make sense.
Sandbox is the mysterious, possibly non-existent web purgatory, in which a new or penalized site is spidered by google bots, accumulates backlinks & page rank but does not appear in google searches for appropriate keywords. If you don’t want to be get trapped in google sandbox then just grow your content and backlinks naturally over time. Don’t look for the quick buck, the quick ranking, or the easy way out.
The Sandbox Effect is the theory that websites with newly-registered domains (or with pages yet to receive a PageRank calculation) are placed in a sandbox (holding area) in the indexes of Google until such time is deemed appropriate before a ranking can commence.