MYLO = Mislynch AND Lose
LYLO = Lynch OR Lose
(Most communities switched the word “lynch” for other words because it offends people, but I CBA to do it for this explanation.)
Basically LYLO is when town is forced to execute mafia lest they lose. E.g., 2v1, 3v2, etc.
MYLO is just LYLO plus one Town. Because (for example) in a 4v2, town has the option to sleep / execute nobody so the next day will become LYLO. (Which, according to some statistics, has a better win rate for town. Makes sense I guess; one person less to think about.)