Officially:
Federal contract information means information, not intended for public release, that is provided by or generated for the Government under a contract to develop or deliver a product or service to the Government, but not including information provided by the Government to the public (such as on public Web sites) or simple transactional information, such as necessary to process payments. Source: 48 CFR § 52.204-21
Unofficially, FCI is the lowest level of information a DoD Contractor has to protect and is only associated with CMMC Level 1.