What is supplying?

When you supply, you receive aToken balances that grow as interest accrues.

Supply APY is the current annualized rate for that asset. It changes with utilization (how much of the pool is borrowed).

Supplied assets can often be enabled as collateral, which is what later lets you borrow.

Earning a rate is not the same as a guaranteed return. Rates move, and the asset itself can change in price.