What is it?
Collateral is supplied crypto that Aave is allowed to count toward your borrowing power. Not every supply is collateral. The reserve must allow it, and the wallet must keep it enabled.
Why does it matter?
Collateral is the cushion under a loan. If its USD value falls, Health Factor falls. Concentration in one volatile asset means one price can dominate the risk.
Example
Maya supplies ETH and USDC, and enables both as collateral. She then borrows GHO. If ETH dumps and USDC stays near $1, the ETH side of the cushion shrinks first.
Where can I find it?
Wallets and My position list supplied assets and whether they are used as collateral. Asset pages show Collateral enabled. Aave events include collateral enable and disable in the live tape when those logs appear.