Human-robot interaction goes both ways. You’ve got robots understanding (or attempting to understand) humans, as well as humans understanding (or attempting to understand) robots. Humans, in my experience, are virtually impossible to understand even under the best of circumstances. But going the other way, robots have all kinds of communication tools at their disposal. Lights, sounds, screens, haptics—there are lots of options. That doesn’t mean that robot to human (RtH) communication is easy, though, because the ideal communication modality is something that is low cost and low complexity while also being understandable to almost anyone.
One good option for something like a collaborative robot arm can be to use human-inspired gestures (since it doesn’t require any additional hardware), although it’s important to be careful when you start having robots doing human stuff, because it can set unreasonable expectations if people think of the robot in human terms. In order to get around this, roboticists from Aachen University are experimenting with animal-like gestures for cobots instead, modeled after the behavior of puppies. Puppies!