MFFT is a hard threshold, not a gradient. Above it, emulsion particles deform and fuse into a continuous film. Below it, they dry as discrete particles — the coating looks chalky, has essentially no mechanical strength and fails every performance test.
It is closely related to Tg but usually somewhat lower, because water plasticises the polymer during drying.
The operational implication is that your line and ambient conditions must stay reliably above MFFT, including the worst case. A water-based coating running fine in July can fail in January on the same line. This is the most common cause of unexplained seasonal quality problems in water-based coating.
MFFT can be lowered by adding a coalescing solvent, at the cost of VOC, or by selecting a lower-Tg polymer, at the cost of film hardness.