Straighten
Straightening is leveling a crooked photo, usually by fixing a tilted horizon or wall. A straight horizon instantly makes a photo look calmer and more professional. It's often the first thing you do before choosing the final crop.
The crop tool has a straighten function: drag the level line along something that should be horizontal or vertical, and the software rotates the photo straight. The auto option often detects the horizon by itself. Keep in mind that rotating crops your image slightly, so leave some margin when shooting. A tilted horizon is one of the most common, and easiest to fix, mistakes.
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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.