On the Ethereum network, internal transactions are transactions that occur when two smart contracts interact. Internal transactions can be thought of as transactions that occur between smart contracts rather than between Ethereum addresses. It is important to mention that these internal transactions do not show up on the Ethereum blockchain. The best way to study internal transactions is by running a full node or using Etherscan looking under the Internal Txns tab.
NFT Term
Internal Transaction
Definition of Internal Transaction from the NFTguild.com glossary.

