An autonomous agent works proactively rather than only responding to a message. Some Copilot Studio agents can be given their own account, which lets them initiate steps as part of a shared workflow. Microsoft's examples are process-shaped: onboarding a new employee, or coordinating the logistics of a recurring meeting.
That is a different mode from the rest of Copilot, where every agent, connector and flow still starts from something a person typed or scheduled. It is the deep end of the pool: try it only once a simpler agent flow or agent already runs reliably. Pilot it narrowly, keep human approval on anything that writes, sends or spends, and watch credit use before letting it run unsupervised on a real business process.