A model's ability to request a function or external service, such as looking up an order or booking a slot, and use the result in its reply.
A language model on its own cannot know an order's status or create a booking. With tool calling, the developer describes a list of available tools to the model. When the model needs one, it asks to run it with specific arguments; the app executes it and returns the result so the model can continue.
This is the mechanism that turns an assistant from a talker into a doer. It lets a store assistant check stock and a clinic assistant show open slots. It is also the foundation of how AI agents work. Practical tip: write a clear description of each tool and when to use it, and validate every argument the model sends before executing. Never give a tool more power than it needs; a view order tool should not be able to delete orders.