A frame buffer graphics device is an idealization or abstraction of your computer's video hardware in order to make programming it simple. It simulates or provides in actual memory a space which maps to the screen byte for byte (or word for word, or 32 bit longword for longword), in such a way that the data at offset 0 stands for the pixel at coordinate (0,0), and so on until the end of line 0 (which has, for example, 1024 pixels), so that byte 1024 represents (0,1), etc.
This makes creating graphics off-screen easy, since you only have to allocate and write to a memory block of the same size as the frame buffer, and then copy the off- screen buffer into the frame buffer.
Frame buffer devices may fulfill more complex tasks, but the thing here is the minimum.