Feet of fines were produced in the course of legal actions that were widely used between 1195 and 1833 to document property transactions. The procedure, the records involved and their interpretation are discussed in A Short Introduction to Feet of Fines by Chris Phillips, which was originally published by the Foundation for Medieval Genealogy.
Numerous abstracts to medieval feet of fines for Northamptonshire can be found on the Notes on Medieval English Genealogy site. In many cases, these are accompanied by links to digital images of the associated records that are held by The National Archives.