Much of Ethereum's trajectory in recent years has focussed on scalability. Rollups are one of the principal methods of scaling Ethereum to accommodate more users and activity without sacrificing security.
Rollups are layer 2 blockchains, forming a second layer on top of Ethereum itself. This additional layer is where the activity on the rollup network takes place, such as interacting with dapps and having transactions processed. Where rollups differ from other networks is that they must periodically submit batches of transactions to Ethereum mainnet for confirmation. This is where their name originates: to create these batches, bundles of transactions are 'rolled up' together.
Submitting numerous transactions to mainnet simultaneously, rather than one-by-one, results in considerable gas fee savings. This is why rollups can be considerably cheaper for the user. You aren't paying for the computation of a single transaction, but are instead paying a small fraction of the cost of computing a larger group of transactions.
There are two principal types of rollup:
- Zero knowledge.
Linea is a zero knowledge (zk) rollup. When it submits bundles of transactions to mainnet, Linea code also supplies what's known as a validity proof. The validity proof verifies that the transactions included in the rollup match their proposed effects, and that users' accounts balances are right to be updated accordingly.
You can read more about zk-rollups here.