What is the "Sandbox Theory"?

[seointerest]

The sandbox theory proposes that Google has a unique ranking element in their algorithm that has been affecting sites whose major SEO efforts began after March of 2004 (this would include all sites registerd after this date). The criteria, elements and solution of the 'sandbox' have not yet been identified or publicized by the SEO community, but ongoing discussion and testing have determined that the two of the most likely issues creating the sandbox could be:
• Age of backlinks - older backlinks now carry greater weight at Google
• SEO specific filters - Google is actively attempting to filter sites out of their index that are actively optimizing (link-building, etc) or appear to have un-natural attributes (optimized pages, links, anchor text, etc.)

[jennilee201]

The sandbox is essentially a filter that was created by Google to avoid spamming of search results by unscrupulous webmasters.

Before the sandbox was created, most new sites could rank pretty quickly, which allowed certain "black hat" SEO professionals to make a quick buck by ranking a new site on the top of Google's index for a very competitive keyword, and then cashing out before Google caught up with their spam technique. These domains typically were registered only for a short period, exploited some loophole in the Google algorithm and relied on sudden bursts of traffic over their limited life span.

By creating an ageing delay which prevented most new sites from ranking immediately for competitive keywords, Google rather effectively decimated the breed of webmaster who sought to build sites with a short-term gain in mind. Many skeptics have questioned the existence of the sandbox in the past. However, with its last patent issuance, Google has almost admitted that a filter has existed that prevents relatively new websites from ranking.

[PaulSmith1]

Sandbox is a box where websites are put for a period of time if the website doesn't qualify.

The logic is that webmasters are given more time to build their web site search engine friendly for better visibility in search engine.