Sandbox is google term. That mean the status your web site is indexed but not in the SERP due to the age of the web site.
Actually, there is no proof until now regarding the truth about sandbox. Sandbox is just a theory given by SEO Guru wherein newly sites are waiting to be filtered by google.
I'm in the sand box. There are only 1-2 pages of relevent google results for the search term "Nam's Bits" but my page most relevant to that term is often omited from the search results. Other pages I have, that are less relevant to that term, are getting close to page 1. It's very odd because most relevant page started making SERP page 1, then started going missing. When it goes missing I checked the first 1000 pages of search results and it's not there.
Yesterday two of my pages related to that term were on the second page of google and had been there for a couple of weeks. This morning none of my pages related to that term show up in the top 10 pages. The thing that "started" the problem was getting 14 backlinks to that term in the span of a week. First that page went up and then started not showing up at all. I think I did to much in too short of time. Here's the best part. Starting on the third page, forum pages with my signiture (which backlinked my page of interest) start showing up.
The sandbox is where bad websites are put in timeout so they can learn not to build links too quickly.