Dynamic range
Dynamic range is the difference between the darkest and lightest parts in which a file still holds detail. The greater the dynamic range, the more you can pull from shadows and highlights when editing. RAW files have a much larger dynamic range than JPEG.
In high-contrast scenes (like a bright sky over a dark landscape) dynamic range decides whether you can get both the sky and the foreground looking good. Lifting shadows or pulling highlights back uses that range. If it falls short, your sky blows out or your shadows crush: then HDR merge helps by combining multiple exposures. Shoot in RAW to keep the maximum dynamic range available for your edit.
Read also the in-depth explanation: Dynamic range explained.
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