There is no official definition from Google yet about their Google TrustRank (nor is there ever likely to be one), but they did just register the trademark for the name “TrustRank”. They might be introducing it to the public in the near future, and perhaps the introduction of the Sandbox is just an extension of the TrustRank implementation.
With this new system, Google will only give high search engine positions to those websites who are deemed trustworthy and legitimate. The system will also seek to eliminate any websites using blackhat or spamming techniques to get a higher search rank.