Rollup
In short A rollup is a Layer 2 that executes transactions off-chain, then posts their data and a proof or claim to a base chain, inheriting much of its security.
A rollup is a scaling design where transactions are executed outside the main chain, rolled up into batches, and posted back to a Layer 1 such as Ethereum. The key idea is that the transaction data lands on the L1, so anyone can reconstruct the rollup's state and check it. That is what separates a rollup from a sidechain, which has its own independent security.
There are two main types. Optimistic rollups assume each batch is valid and give a challenge window, often about seven days, during which anyone can submit a fraud proof to reverse a bad state. ZK rollups attach a validity proof, a piece of cryptography that proves the batch was computed correctly, so the L1 can verify it without waiting. In both cases a sequencer usually orders transactions and builds the batches.
For holders, rollups mean much cheaper and faster trading than on the L1 while keeping a credible path back to it. They also bring trade-offs. Standard withdrawals from optimistic rollups to the L1 take days, which is why fast third-party bridges exist and add their own risk. Many rollups still have a centralized sequencer and upgradeable contracts controlled by a multisig, so security in practice depends on how far the project has decentralized.
Check whether a rollup's proof system is actually live and permissionless, who holds the upgrade keys, and whether there is a forced-withdrawal path if the sequencer stops. Remember that a token deployed on a rollup is its own contract, so verify the address on that network specifically. Lower fees also make it cheap to launch tokens, so the usual scam checks apply just as much there.