Everybody knows that a blockchain consists of chains of data.
A sidechain is essentially something that runs in parallel. You have the mainchain and sidechain.
Let’s say it has a two-way peg, and assets can flow from the mainchain to the sidechain and back to the mainchain. Those assets are basically controlled by some type of timing mechanism, like an atomic swap or a smart contract.
Assets from the mainchain go into a smart contract, they’re locked/verified. Then they’re released onto the sidechain.
A famous sidechain like RSK. So, Rootstock which is a sidechain for Bitcoin, that basically wants to create smart contracts on Bitcoin.