A "Rollup" is a type of blockchain solution designed to improve transaction efficiency and reduce costs. It works by bundling multiple transactions into a single batch, which is then processed on the main blockchain. This helps to alleviate congestion and lower fees, making it easier for users to conduct transactions. Rollups can be categorized into two main types: Optimistic Rollups and ZK-Rollups. Optimistic Rollups assume transactions are valid and only check them if challenged, while ZK-Rollups use cryptographic proofs to verify transactions instantly. Both methods aim to enhance scalability and speed for decentralized applications on platforms like Ethereum.