My understanding is that fully flexible fares are not only changeable/refundable, but also have no advance purchase requirements or other restrictions. So if you want to make a change, there generally won't be an additional charge for flights on route right up until departure (well, I guess assuming there is still bucket inventory for the fare on flights).

Flexible fares are also changeable/refundable, but may have advance purchase requirements or other restrictions like roundtrip booking requirements or blackout dates. So if you want to make a change to flight on booked route, there will likely be fare differences involved the closer you get to departure due to no longer meeting advance purchase requirement of originally purchased fare or if you have roundtrip fare and no longer meet minimum stay requirement, etc. Naturally, the flexible fares will be cheaper than the fully flexible fares. Additionally, as noted above, they may have a fee for cancellation, but generally do not from what I have seen (any fees for cancellation will be listed in Penalties section of fare rules). Generally speaking, if you are mainly interested in the ability to cancel for refund prior to departure, there's not much reason to spring for the "Fully Flexible" fares.

Here's a section of fare rules from a NVFHZRU3 "Flexible" main cabin fare on DTW-LAX (booked as part of roundtrip) which has a 21-day advance purchase requirement and also has roundtrip booking requirement (with Minimum stay section) and blackout dates (see below). "Fully Flexible" fares on same route book into B class (B0AHZRU1 fare) and do not have an advance purchase requirement or min stay/black out dates.