Natural language
Natural language is ordinary human language, like English or Spanish, as opposed to code.
What it tells you about a developer
A developer who has shipped a natural language interface has worked on the seam between a model and the rest of a product. That is applied AI product work, and it pairs a model with ordinary application engineering.
Natural language means the words people normally speak and write, rather than a programming language. When a product lets someone type a plain sentence instead of clicking through menus or writing code, that is called a natural language interface. The name separates it from the strict, narrow syntax a programming language requires. A developer who works on natural language interfaces builds the part of a product that turns a plain sentence into something a computer can act on.