Function calling is the mechanism that lets a model trigger a piece of software in a tidy, structured way. Rather than just writing a sentence, the AI produces a clean request, such as 'get weather for Paris on Friday,' that another program can act on. It is the plumbing that turns an AI's intentions into real actions inside an app.
For example, You say 'remind me to call Mom at 6,' and function calling passes the time and message to the reminders app.