A zero-knowledge rollup (zkRollup) is a type of Layer 2 scaling solution on Ethereum that ensures a much higher throughput and much lower costs without sacrificing security.
It does this by bundling up hundreds of transactions into one and moving them off-chain. There, it generates a cryptographic validity proof and sends it back to the main blockchain.
The fact that Ethereum verifies these proofs, and stores enough data to tell what off-chain account owns what, means that zkRollups inherit the same non-custodial security as Ethereum, but can run much faster.
For more in-depth technical info, read our Design Doc.
Additional learning material