In plain words
In an AI system, tools are callable functions or services that extend what a model can do. Examples include web search, code execution, database lookups, and calendar actions.
A closer look
The model receives descriptions of available tools and their expected inputs. It can request a tool call with arguments; the surrounding application or provider runs the operation and returns a result. The model can then use that result to answer or choose another step.
A tool is the capability; a tool call is one request to use it. Function calling commonly refers to this structured interaction. Some tools only read information, while others change files or external systems. Access and permissions are controlled by the software that runs them.
In practice
An assistant answering a weather question calls a weather service with the city name, receives the forecast, and uses that result in its reply.
A useful distinction
Mentioning an action does not mean a tool ran. Tools do not automatically make answers correct: the model can choose the wrong inputs or misinterpret the returned information.