Reference view
With the reference view you place another photo next to the one you're editing in Lightroom, as a benchmark. So you match the exposure, white balance and colour of one photo to the other. Essential for getting a series consistent. You activate it with the key R in the Develop module.
You drag your reference photo into the left pane and edit on the right until both match. This is ideal if the light changed during a shoot and all the images still need to look the same. It also works nicely for editing toward an earlier-edited key photo. Tip: pick your best-edited photo as reference and bring the rest of the series toward it, so your look stays consistent.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.