That means there is no problem related to hardware ( motherboard, gfx card, RAM, etc. ) and your computer should continue loading your OS.
The single short beep means the POST (POwerup Self Test) passed. This includes the CPU, PSU, basic functions of the motherboard, basic functions of the RAM, and the interface to the video card.
It cannot do a detailed check of the RAM the way a program such as memtest86+ can. It cannot do a detailed check of the video card itself. If you have something wrong with the rendering or out put circuits, it cannot check it. That is why you can successfully pass the POST and still have something wrong with the video card.
It does not check the keyboard, mouse, monitor, or drives. A motherboard sitting on the bench with only CPU & HSF, RAM, PSU, and video card plugged in will pass the POST.