Post-crop vignette
A vignette darkens (or lightens) the corners of your photo to draw the eye to the center. When editing, you add this deliberately as an effect, separate from the vignetting your lens sometimes produces on its own. A subtle dark vignette instantly gives a photo more focus and mood.
In Lightroom you'll find the post-crop vignette in the effects panel, so it stays neatly around your image even after a crop. Keep it subtle: a slightly dark edge works, a heavy black tunnel quickly shows. A light vignette can actually enhance a high-key or airy look. For more precise work, use a radial mask with negative exposure, so you control the shape and placement yourself.
Read also the in-depth explanation: Post-crop vignette 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.