Well, look at a it the same way you would look at any other compound word, by breaking it down into it's constituent parts:
Frame: A completely rendered image.
Buffer: A temporary storage area for data being transmitted between two devices that function at different speeds.
So a frame buffer is a temporary storage area where fully rendered images (frames) are held until they are able to be displayed on the monitor. In terms of gaming, you can think of the entirety of the VRAM as functioning as a frame buffer.