GLFW | What is a frame? Here I was reading about buffers and found such text: 


  When the entire frame has been rendered, the buffers need to be swapped with one another, so the back buffer becomes the front buffer and vice versa.


And also this 


  The swap interval indicates how many frames to wait until swapping the buffers, commonly known as vsync.


The question is: what is a frame? And how does it differ from buffer?
Conceptually, a frame is the image you're trying to create, presumably for the purpose of display. It is the product of your entire rendering operation for a specific slice of time.

A "buffer" is the memory for the image you either are trying to create or have already created. Hence the common term: framebuffer. Indeed, the choice of the word "buffer" is because you are typically buffering multiple images. That is, one buffer will currently be in the process of being displayed, while another buffer is being rendered.