Antenna switch.

If you have 2 or more CBs at the same location, but only use one at a time, a special switch may be used to change which radio is connected to that antenna.
BEWARE: Only one radio has a load connected, so transmitting on the wrong radio could cause serious damage.

The alternative to a switch, is a quick connect plug.
This plug has no threads on the inside so, rather than being screwed on, this plug just slides on. The main use of this would be for someone testing many radios, such as in a repair environment.

BEWARE: You must remember which radio has the antenna connected, as transmitting on the wrong radio could cause damage to it's output. Problems can also occur if the connections are not tight.