The "Apps Search" lab feature extends search with Google Docs and Sites results. It will find the most relevant docs and sites and show them below the Gmail search results. The feature is definitely interesting and will allow users to quickly find documents and pages from the other services.
However, the feature does not seem to be working fully right now as my searches did not return results from my Google Docs account. This might be because Gmail might take some time to index all the files in the Google Docs and Sites account.
To enable the feature go to your Google Labs tab in Settings (click here) and scroll down till you see a header for "Available Labs". Under this header enable the "Apps Search" feature and save your settings.
The next time you use Gmail to search for something, you will also be show results from your Google Docs and Google Sites account.