TL;DR: A rollup is a Layer 2 scaling solution that executes transactions on a separate chain and then "rolls up" hundreds or thousands of them into a single compressed batch that is submitted to Layer 1 for verification and permanent storage. The rollup handles the heavy lifting of transaction execution, while the L1 provides security by verifying proofs and storing transaction data. This architecture delivers dramatically higher throughput and lower fees while inheriting the security guarantees of the underlying L1. The two main types are optimistic rollups (which assume transactions are valid unless challenged) and ZK rollups (which provide cryptographic proofs of correctness).
The Simple Explanation
Think of a rollup like a teacher grading exams. Instead of the principal (L1) grading every single exam individually, the teacher (rollup) grades them all, compiles the results into a summary report, and submits the report to the principal for verification. The principal checks that the report is accurate (either by trusting the teacher unless someone objects, or by verifying a mathematical proof), then files it as the official record. The principal only needs to review one summary instead of hundreds of individual exams, dramatically reducing the workload.
In blockchain terms, users submit transactions to the rollup. The rollup executes them, computes the resulting state changes, compresses everything into a batch, and posts the batch to L1 along with either a fraud proof mechanism (optimistic) or a validity proof (ZK). The L1 verifies the batch and stores it permanently. One L1 transaction now represents hundreds or thousands of L2 transactions.
How Rollups Work Under the Hood
The rollup lifecycle follows a clear sequence. Users submit transactions to the L2 network, where they are picked up by the rollup's sequencer. The sequencer orders the transactions, executes them against the current state, and produces L2 blocks. Periodically, the sequencer compresses a batch of L2 blocks into a compact representation and posts it to the L1 chain.
