Loop
This action starts an unnamed loop of all the channels your client is joined to.
For each channel, the Channel > On joined channel loop condition is triggered, with that one channel selected, so you can use it in any action, condition or expression that refers to the selected channel.
By default, the channel list in order of first to last joined channel.
Once the loop is over, the Channel > On joined channel loop finished condition is triggered once.
All those loop events happen instantly. Read more about how loops work here.