When the property was registered in the name of your late father's ex-wife, your father indicated that he was making a claim to have an interest in the property.
The caution does not prove anything, either way, about whether your father had an interest in the property. It simply serves to warn anyone who is thinking about buying the property that the vendor might not be the outright owner of the property.
Information about cautions, and their effects, is embodied in Chapter 2 of the Land Registration Act 2002:
http://www.opsi.gov.uk/acts/acts2002/20009--d. htm#15
(If the caution was registered prior to this Act coming into effect, it would have been under the provisions of the 1925 Act. This has now been repealed and it's the 2002 Act which determines the methods by which a caution can be removed).
Chris