A sidechain is a secondary blockchain that runs parallel to a main blockchain, allowing for the transfer of assets and data between the two. This setup enables developers to create specialized features or applications without congesting the main blockchain, improving efficiency and scalability. Sidechains can operate under different rules and protocols, making them flexible for various use cases. For example, a sidechain might focus on faster transaction speeds or support smart contracts, while the main blockchain, like Bitcoin or Ethereum, maintains its core functionalities and security.