GLFW | What is a frame?
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.