The sandbox is a probationary period for new sites that affects their ranking in Google. Newer sites tend to rank poorly compared to where you would expect them to rank based on link popularity and other factors. The consensus is that the sandbox lasts from 9 to 12 months.
Matt Cutts is on record as confirming that there is a probationary period before new sites rank well in Google.
This is what is commonly called the “Google sandbox effect”.
The Google sandbox effect is clearly defined in a number of reputable SEO forums and blogs.