A foreign function interface (FFI) is a way for a computer program written in one language (like C or Java) to interact with another language (like Python or Ruby). It allows two different programs written in different languages to communicate with each other and share data or resources. Think of it like two people speaking different languages. If one person speaks Spanish and the other person speaks Portuguese, they can communicate by using an FFI. The FFI acts as a translator between the two programs, allowing them to share information.