Compared to sidechains, layer 2 rollups are considered by many developers to be the more secure way for Ethereum to scale. Rollups, like sidechains, take the stress off of Ethereum by executing transactions on a separate blockchain. Transactions are “settled” on the rollup chain, bundled up, and then “posted” to Ethereum. Unlike sidechains, rollups borrow their security from the Ethereum mainnet – they use fancy cryptography and other methods (like a multi-day dispute period) to allow Ethereum mainnet nodes to confirm that transactions are legitimate.
