Developers and Apple create these signatures by running the checksums before the applications are distributed and include them with the program. To protect application integrity, Apple includes various checksum routines, which simply treat any downloaded file as a lump of raw binary data and perform summations of the bits in this raw data according to a specified algorithm (MD5, SHA1, etc.), after which you get a large number that represents a unique characterization of the file's integrity. The error message says that the product distribution file could not be verified.
MacFixIt reader Silverio recently wrote in with such a problem: I've been experiencing problems to install updates for iMovie through Apple Store. Though in most situations the Mac App Store works fine to download and run a specified installer, there are times when errors occur and even persist regardless of how many times you retry the download. Apple's Mac App Store, which was introduced in OS X 10.6.6, has become Apple's central location for distributing new software as well as managing updates to the OS and purchased applications.