A sidechain is a separate blockchain that is attached to a main blockchain, allowing for the transfer of assets between the two. This setup enables users to take advantage of different features or functionalities without congesting the main chain. For example, a sidechain can offer faster transaction speeds or lower fees, making it an attractive option for specific applications. Sidechains are often used in conjunction with Bitcoin or Ethereum, allowing users to move their tokens to the sidechain for various purposes, such as testing new features or conducting transactions more efficiently. This flexibility enhances the overall ecosystem of blockchain technology.