A checksum was a procedure by which an individual could determine if a computer file had been corrupted by comparing two sets of data for discrepancies in total aggregate amounts of data bits stored.
In 2364, upon discovering the confession of Captain Jean-Luc Picard for mistakenly destroying an unarmed starship at the Battle of Maxia by confusing a subspace antenna for a weapons cluster aboard the USS Stargazer, Commander William T. Riker ordered Lieutenant Commander Data for an evaluation of the authenticity of the confession. Data compared the total number of bits of the logs contained in the main computer core to those found in the personal logs of Captain Picard, which resulted in checksum discrepancies being discovered. Data concluded that one of the two had been forged by the Ferengi.
It was later concluded that DaiMon Bok had forged the confession in an effort to discredit Captain Picard, whom he blamed for the death of his son in 2355. (TNG: "The Battle")