Interoperability in the crypto sense is covered in full under Blockchain Interoperability, which explains how separate blockchains exchange tokens and messages and the risks involved. This page gives a short definition of the wider term and explains how it is used.
In general, interoperability is the ability of different systems, products or organizations to work together without special effort from users. In payments, it means that a card works at any merchant on the network, or that a customer of one bank can pay a customer of another instantly. It depends on shared standards, such as message formats and application programming interfaces, and on agreements about rules and liability.
In crypto, blockchains are built as separate networks that cannot read each other's records by default. Interoperability tools let assets and data move between them, through blockchain bridges, protocols such as Inter-Blockchain Communication (IBC) and other cross-chain systems. Because many large hacks have targeted bridges, how interoperability is secured matters as much as whether it exists, which is why the topic has its own entry on this site.