what is does is simply setting the selected object into the "locked" state so if it is a ui object you can use it as if the patcher was locked (i.e. you can change the value of a number object by clicking and dragging) while the rest of the patcher remains unlocked. With the exception of bpatcher you can achieve the same by cmd+click (ctrl+click for windows) so to speak "on the fly".
Thanks for clarification. It would be obvious (but otoh distracting) if cmd/ctrl + clicking an object also gave this green highlight. For me cmd+click works also for bpatcher.
I see this feature could be useful, but why there is the limitation to lock only one object at a time?