A layer 2 scales a base chain by executing transactions off the L1 while inheriting security via proofs or fraud challenges.
Optimistic and ZK rollups are common Ethereum L2 designs. Users bridge assets to L2 for cheaper activity.
In research workflows, Layer 2 is usually interpreted with Optimistic rollup, ZK rollup, Bridge, and Ethereum rather than as a standalone signal. Throughput, finality, and fee markets determine user experience and where activity migrates during congestion.