It’s two BIP proposals (BIP300 + BIP301) that enable a decentralised 2 way peg from side chains to mainchain bitcoin.

So you can various bitcoins sidechains with various designs. You copy the code from Monero or ETH or whatever shitcoin but then add it to a bitcoin sidechain/layer 2. So bitcoiners get the benefit of extra functionality like privacy, defi, and scalability without the shitcoin.

The transaction fees on the sidechain go to bitcoin miners via merged mining originally created by Satoshi with Namecoin in 2011.