The Smilers are androids, mounted in booths all across the space faring version of Britain that left Earth in the 29th Century. They let people know what's good and what's bad. The terrified population keep them clean. Their heads can rotate to show three faces: happy; warning or very dangerous indeed.
And if things get very dangerous, the Smilers can leave their booths and move around. They're actually androids, an extension of the 'starship's' security system.