What is it?
aTokens are the receipts Aave gives suppliers. If you supply USDC you receive aUSDC. The aToken balance grows as interest accrues, so you do not have to claim interest in a separate step.
Why does it matter?
When you read supplied token amounts, you are usually looking at this growing balance valued at the oracle price. It is still a claim on the pool, not a separate bank account at Ronnie.
Example
Sam supplies 1,000 USDC and later sees about 1,012 aUSDC. That extra 12 is interest paid by borrowers, not a second deposit Sam made.
Where can I find it?
Ronnie shows supplied USD and token amounts on asset and wallet views. It does not list aToken contract balances as a separate table. The supplied figure is the economic claim.