Before/after view
With the before/after view you compare your edited photo with the original in Lightroom. You can look side by side, top and bottom or in one split image, or quickly toggle back and forth. So you see exactly what your edit has done. You activate it in the Develop module with the key Y.
Very useful for checking you haven't overshot, something that happens easily while editing. With the backslash key you switch between before and after at speed without splitting the view. The split view is nice for comparing one part of the photo directly. Tip: look at the 'before' regularly, then you keep an eye on the difference and avoid an over-edited look.
Related terms
Prefer a look in one click?
The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.