If a programme or a person is on/off (the) air, they are/are not broadcasting on radio or television.

The radio station is on air from 6 a.m. 广播电台早晨6点开始播音。

As soon as the war started, any broadcasts with a military theme were taken off the air. 战争一爆发，所有有关军事主题的节目都停止播出了。

We worked very hard to get a conversation about climate change on the air.

Unfortunately, the best part of his interview was off air.

The state broadcaster went on air immediately to announce the invasion.

See also: air noun (BROADCAST)