A ZK rollup generates a succinct proof, verified cheaply by a contract on the base chain, that the batch it posted follows the rules. Correctness is established immediately rather than assumed.
The practical benefit is exit speed: withdrawals settle once the proof is verified, in minutes to hours, instead of the seven days an optimistic-rollup requires. Costs are proving-heavy, which historically made general-purpose ZK rollups harder to build.
Be precise about the language. "Zero knowledge" here mostly means succinct proof of correct execution, not privacy. Most ZK rollups are fully transparent; your activity is as visible as anywhere else on-chain.
Related: rollup, optimistic-rollup, layer-2, finality