An external move is an active action that can be completed outside of a Pokemon Battle. Moves like this often require a specific move to be known by the Pokemon for it to actually be used. But every Pokemon comes with the known move Forage.
When you sent a Pokemon out of it’s ball, the external move dashboard will appear in the top right corner. In control settings you can change your binding key for activation but the default key is “G”. If your Pokemon knows more than one External Moves, you are able to cycle through them with the default key being “B”.
A Cool-down timer will consist once an External Move is executed, which will actually un-saturate the icon. Different External Moves will have a different timer duration and can also change on the Pokemon stats. Even if the Pokemon is withdrawn then placed back out, the cool-down will still persist.