MDF is minimum defense frequency.
Its the frequency of your range you have to continue with to stop villain from being able to profitably bet his entire range.
If, for instance, villain bets pot sized on the river then you have to continue with at least half the hands in your range either by calling or raising in order that he cannot bet any two cards and make money with the bet. If you call with exactly the MDF (when villain bets the pot that is fifty percent of your range) then you keep him from doing better than breaking even with his bluffs. If you fold any less than the MDF then your opponent should bluff with all his hands that don't have any real value. If you fold less than MDF then villain cannot auto profit with his bluffs and, in fact, he should not bluff if you call with greater than MDF.
If you call with exactly the MDF and he uses exactly a 2 to 1 value to bluff ratio then you have made him indifferent to bluffing and giving up with his weak hands and he has made you indifferent (breaking even/doesn't matter) between calling and folding with all of your hands that beat his bluffs but lose to his value bets.