That there is a computer at ATC that a human looks at, reads what it says with their eyes, speaks those instructions over the radio in a specific protocol, another human listens to it (and confirms within that protocol), and inputs those control signals into the airplane.

Computer -> human -> radio(spoken protocol) -> human -> plane.

There aren't a lot of practical reasons it can't just be

Computer -> radio(digital protocol) -> plane

(There are nonzero reasons, such as the presence of weird situations, VFR aircraft, etc., but it's not a lot.)