Ki means a demon. This kanji also reads ONI which is a Japanese demon. Oni is a human style demon with horns on its head.
Typical Oni has red skin, two horns with yellow and black stripe, and a big mouth full of sharp fangs. And wears a pant made of tiger skin.
Metsu means to destroy and eliminate something. Not only defeat it, but there’s a nuance that trying to wipe out something completely out of this world.
Yaiba is a blade or edge of a sword. So Kimetsu no Yaiba means the sword for elliminating demons = Demon Slayer.