Glossary
Rollup
A Layer-2 scaling design that executes transactions off the main chain and posts the data (and a validity or fraud proof) back to it.
A rollup batches many L2 transactions, executes them off-chain, and "rolls them up" into compressed data published on L1. The distinction between optimistic and ZK rollups is how the L1 becomes convinced the batch was processed correctly: optimistic rollups allow fraud proofs after the fact, ZK rollups attach a validity proof up front.
Either way, the security model is "as good as the L1" plus correctness of the rollup contracts — much stronger than a sidechain with its own validator set.