What it means
Normally, an LLM just outputs text. Function Calling allows it to output a structured command that a computer can execute. It can say 'GetWeather(London)' instead of just saying 'I think it is raining.'
Why it matters
It turns a chatbot into an agent. It allows AI to actually do things-like book a flight, query a database, or send an email-rather than just chat about them.