The poker term MDF is an abbreviation for Minimum Defense Frequency. This relatively novel poker term describes how often you need to defend against an opponent’s bet or raise to make their bets neutral.
The MDF dictates what portion of your range you should be continuing within a given situation to play GTO and be unexploitable by bluffs.
In other words, this number tells you how often you need to call or raise a bet to ensure that your opponent isn’t making a profit by always bluffing you in a certain spot. This is a relatively high-level concept that you’ll need to master as you move up the stakes.