Program means the camera will choose the aperture and shutter setting for you. When in Program mode the camera usually keeps the shutter speed between the flash sync speed of the camera, and the manufacturer's suggested minimum hand holding shutter speed. In the past this would typically be 1/60th or 1/125th, although most SLRs/DSLRs today probably can tell what the lens focal length is, and set the minimum hand held speed to 1/focal length.
Program was the first full auto exposure mode. It's been around since at least the 70s, and maybe longer? The auto exposure modes we are familiar with such as portrait, landscape, action, etc... were introduced in the 90s. They offer the photographer a bit more control in how the camera will choose the settings. Portrait mode is Program that will try to give shallow DOF. Landscape mode is Program that will try to give deep DOF. Action mode is Program that tries to give a high shutter speed.