This term was the talk of Libraryland in the late 80s and 90s as many libraries moved from manual inventory control to automated library systems. The recon was the process of taking the union card catalogue and converting it to machine readable cataloguing. In the early days I imagine there was a much larger proportion of shelf cards that had no equivalent record in an accessible database. Now, not so much – most records would be available for importing into an automated system.
A more detailed description from The Colorado State Library.