The documentation explains it.
getHeight return the size of the client area (this is generally the usable portion of your game window).
getBackBufferHeight return the size of the frame buffer, where actual drawing happens before it's presented to the window.
Sometimes these values differ; for example you might render to frame buffer that is smaller than the window and let presentation to the window upsample it, to achieve a "pixelated" effect. Similarly you might use a larger frame buffer and let it downsample to achieve a crude anti-aliasing effect.
You can look at the source to verify this; note that the specifics of the behavior of the functions differ based on which backend ends up being used. For example, compare this implementation with this one.