It means that if for any reason the cylinder gets "hard" to revolve, because it is dirty, fouling, bullet cases blew back, etc, then you are out of luck with a hammerless - it wont fire.
A hammer on a revolver means that you are much more likely to force the cylinder to revolve by pulling the trigger back and going to single action.
I got rid of all of my hammerless revovlers after one of them gave me trouble.