Tone mapping
Tone mapping is bringing a very large tonal range (from an HDR merge, for instance) back to what a screen or print can show. It distributes the tones so both the light and dark areas keep detail. Done well it looks natural, overdone it gives that typical 'HDR look'.
After merging multiple exposures into an HDR, you need to tone map the result to make it usable. Keep the edit subtle: don't open local contrast and shadows too far, or you get grey, unnatural skies and halos around contrasts. In Lightroom the HDR merge produces a DNG with extra leeway that you simply edit with the normal sliders, which often looks more natural than heavy tone mapping software. Aim for a result that looks like what you saw yourself, not like a painting.
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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.