Contract Application Binary Interface. Have you heard about this? If you have been learning Solidity and have created an ERC-20 token or ERC-721 then this term has come up.

What are ABI's exactly??

Any contract that you create is associated with an ABI. When you compile a contract whether you use Truffle or Hardhat, you end up producing an artifact file, within that artifact file lives the ABI. ABI's are the standard way of interacting with a contract. This is for interacting within the ethereum ecosystem which, includes outside of the blockchain and contract-to-contract.

You can use Etherscan to find the ABI for a specific contract by entering the address of the contract.

I thought it would be helpful to talk about ABI's since this will be a part of every contract. If you would like to learn more checkout Solidity docs for more details.