Variable frame rate is a term in video compression for a feature supported by some container format

Container format

A container or wrapper format is a meta-file format whose specification describes how different data elements and metadata coexist in a computer file....

s which allows for the frame rate

Frame rate

Frame rate is the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...

to change actively during video playback, or to drop the idea of frame rate completely and set individual timecode for each frame. VFR is especially useful for creating videos of slideshow presentations or when the video contains large amounts of completely static frames, as a means of improving compression rate.