Histogram
The histogram is the graph that shows how the tones in your photo are spread, from black on the left to white on the right. When editing, it's your most reliable gauge: your screen can lie, the histogram can't. Peaks against the edges mean loss of detail.
Keep an eye on the histogram while you adjust exposure, highlights and shadows. If the graph piles up against the right edge you're blowing out highlights; against the left means crushed shadows. A nicely spread histogram without clipping uses your whole tonal range. Note: there's no such thing as a 'perfect' histogram, an evening shot should be dark, so use it as a guide, not a law.
Read also the in-depth explanation: Histogram explained.
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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.