An autonomous drone can combine navigation, sensing and flight control to follow an authorized mission and respond to operating conditions. Autonomy varies by system and does not imply that every decision is independent of human supervision.
Reliable service depends on the aircraft and the wider operation, including communications, charging, maintenance, ground facilities and validated responses to failures. Demonstrating one flight does not establish readiness for sustained deployment.
ELI5
An autonomous drone is a flying machine that can carry out parts of a trip using its control system instead of needing a person to steer every moment. People still define its job and the conditions in which it may operate.
For example, a delivery drone could follow an approved route to a receiving point and return for charging. A complete service also needs a way to check the aircraft, handle problems and keep people safe.
Does autonomous mean there is no human responsibility?
No. Operators still define missions, supervise the service and manage operating limits.
Is flight software the whole system?
No. Hardware, maintenance, infrastructure and safety procedures are essential parts of reliable operation.

