U+1F525 · emoji since 2010
The fire emoji means something is hot in the best possible way. People drop it on a sharp outfit, a song on repeat, a great photo, or anyone who is absolutely killing it.
It still works literally when you're talking about heat, spice, or actual flames, but online it's mostly a compliment. It's short, punchy, and very easy to overuse, which is half the fun of it.