The Google Sandbox is nothing but it is very similar to a new website being placed on probation, and kept lower competitive keywords expected in searches, prior to being given full value for its incoming links and content.
Google Sandbox is a Google Algorithm which keep tracks of the methods you are trying to build backlinks for you blogs. This will track the IP address of your computer and will notice how frequently and how much backlinks you are building for your blog either they are blog comments or guest blogging.
The Sandbox or the Google penalty is a name given to an observation of the way Google ranks web pages in its index. If a website is not following the Google guidelines, then they will put this website in sandbox.
not sure if the sandbox even exist anymore.. I have build and ranked websites on page 1 of google in weeks, where at onetime you just could get there (sandbox) before... had to wait and earn it I guess..