Meaning
A bright red face dripping with sweat, tongue hanging out and eyes strained, like someone melting in a heatwave. Literally it means overheated: scorching weather, a tough workout or food that is far too spicy.
That literal use is perfectly safe: “It’s so humid I’m melting just walking to the bus stop 🥵” The other common meaning is that someone looks “hot,” so reacting with it to a person’s photo signals attraction. Think about which reading the recipient will pick before using it on a coworker’s beach picture.