ABI is the acronym for Application Binary Interface
An Application Binary Interface is the machine readable version of code that is parsed between processing devices on different levels. Computational devices (phones/computers/TV, etc.) have a layer of communication that must happen between the input (software) and the input (Hardware); ABI’s dictated how those two interfaces will interaction through bytecode/ binary code/ opcode/etc.
In the case with blockchain and cryptocurrency, a great example of an ABI can be seen with the Ethereum network. Etheruem is a “global computed”, everything that happens within its network must be processed by devices that are connected to it. So whenever activity happens, such as a smart contract/ DAPP is interacted with, Ethereums VM must relay that interaction between the network and the device(s) on which it will be processed, this is done using an ABI.