It's a token protocol invented by the same fuckstick responsible for ordinals. It's supposedly "better" than BRC-20 or RGB (both themselves just degeneracy promoted by pepple who hate bitcoin), because all balance data is maintained in UTXO's (instead of off-chain). Here's Casey Roadarmor, the inventor of runes, talking about what a great idea Runes are:

"I'm not sure creating a new fungible token protocol for Bitcoin is a good idea. Fungible tokens are 99.9% scams and memes."