lmao means the sentence, "laughing my a$$ off" usually used in a funny type, similarly to "lol" it is more used by a mature audience. But kids use it whatsoever.
"lmao!! The movie about (random noun) was so funny!"
Someone overly obsessed with someone, or just really likes someone
Zenitzu: I LOVE YOU NEZUKO CHANNNNNN
Tanjiro: what a simp.
Nezuko: mhm ð