Q:

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?

A:

What is a frame?

This word comes from the film industry, where the images on a reel of film are referred to as frames.

In filmmaking, video production, animation, and related fields, a frame is one of the many still images which compose the complete moving picture.

And how does it differ from buffer?

A buffer is a region of computer memory allocated for storing data. The image that is generated by rendering a scene is stored in a buffer.
While a still image can be represented by a single buffer, an animation requires many buffers to be presented in a sequence (like a reel of film). Each buffer in that sequence can be considered a frame when given this context.

