MFL and FFL fittings are simply a type of compression. The M in MFL and the F in FFL simply stand for Male and Female. The beauty of flared fittings is the seal that is created when the fittings are tightened as the seal is not created by the thread, but by the contact between the two flared (or chamfered) surfaces. The male and female parts are generally made with a 45 degree flare giving the connection a solid contact surface and prevents the need for thread tape or sealant.
Note that these fittings are generally referred to as 1/4" MFL and 1/4" FFL, however, the 1/4" designation is not 100% accurate - the actual thread size of these fittings is 7/16"-20UNF. View the MFL diagram below.
