Introspection opcodes allow for a more modular approach to covenant construction, providing a way for scripts to inspect certain aspects of the transaction.
Introspection opcodes push data from the current transaction onto the stack, unlike ordinary push opcodes (which push arbitrary user-provided data onto the stack) or other opcodes (which manipulate data already on the stack). This allows covenant authors to put conditions on specific parts of a transaction, giving them fine-grained control over how coins may be moved.
A number of introspection opcodes went live on the Liquid Network in November 2021, along with Taproot, as part of an upgrade to Elements. With these new opcodes, developers can deploy advanced smart contracts that offer financial products, such as non-custodial limit orders and fully collateralized options contracts, to traders seeking to reduce the reliance on third-party trust.
See here for the complete list of introspection opcodes added during Liquid's Taproot upgrade.