In short Networks built on top of a base chain (like Ethereum) that batch transactions to cut fees and boost speed while inheriting the base chain's security.
A rollup executes transactions off the main chain, then posts a compressed record back — so hundreds of transfers share the cost of one settlement, and fees drop accordingly. "Inheriting security" is the important claim and it is not automatic: it depends on whether anyone can independently prove the rollup cheated, and on whether users can withdraw without the operator's cooperation. Those two properties, not throughput numbers, are what distinguish the serious designs.