Eye in the sky" refers to camera surveillance in casinos. The cameras are positioned in the casinos in such a way that they not only monitor the players and the gaming tables, but also the hallways, elevators and catering facilities. What makes this special is that many casinos use special cameras that allow Casino staff to zoom or pan the camera.
More specifically, the cameras are often covered by a semi-transparent plastic sphere. This makes it nearly impossible for an outsider to see which direction each camera is facing. Since these cameras are usually located on the ceiling of the casinos, they are also referred to as "eye in the sky". Most land-based casinos nowadays have a large number of high-resolution cameras so that the action at the gaming tables can be completely monitored.
Among other things, this is intended to prevent players from indulging in card counting while playing Blackjack, for example. Furthermore, the "eye in the sky" also serves the security of honest gambling friends. For example, the video data can be backed up in the event of fraud or theft.