Watermark
A watermark is a name, logo or text you lay over your photo to brand your work or guard against unwanted use. You usually add it at export, so your original stays clean. It can be subtle in a corner or bold across the image.
In Lightroom you make a watermark template with your name or logo and set placement, size and opacity. Keep it subtle when you mainly want to sign, or make it larger and central as protection on previews. Save your logo as a PNG with a transparent background for the cleanest edge. Bear in mind a determined user can remove a watermark, so treat it as a hurdle, not a lock.
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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.