What is it?
A reserve is one listed asset inside one Aave market. WETH on Ethereum is a different reserve from WETH on Base. Each has its own supply, borrow, caps, and risk parameters.
Why does it matter?
Almost every asset investigation is reserve-level. Mixing chains will mix different pools and different rules.
Example
USDC on Ethereum can be highly utilized while USDC on Polygon is quiet. Cross-Chain exists so you do not treat them as one pile.
Where can I find it?
Assets lists every reserve Ronnie can read, ranked by supplied USD. Click a symbol to open /assets/[marketId]/[address]. Market pages list the reserves in that pool.