Composable DeFi is covered in full in the entry Composability, which explains how open protocols connect, why the property matters and the risks it brings. This page gives a short definition and explains the name.
Composable DeFi refers to decentralized finance applications that are designed to be combined with one another without anyone's permission. Because smart contracts on a public blockchain can call each other directly, a developer can build a new product by connecting existing ones: for example, depositing tokens in a lending market, using the resulting receipt tokens as collateral tokens elsewhere and routing trades through a decentralized exchange, all in a single transaction. This is why DeFi protocols are often called "money legos". The same links also mean that a bug, hack or price shock in one protocol can spread quickly to others that depend on it.
"Composable DeFi" simply applies the general property of composability to finance. The full entry covers both the general idea and its DeFi uses, and related ideas appear under Composable Token.