Presets are Adobe Lightroom's way of saving a set of editing adjustments so that you can easily re-apply the same adjustments to multiple photos. These adjustments include things like exposure, contrast, highlights, shadows, whites, blacks, and pretty much anything else you can change on a photo in Lightroom.
Essentially, all of these adjustments determine how a photo looks. So in layman's terms, a preset is a pre-configured look that you can apply to any number of photos with one click.
If you're familiar with using filters in Instagram, VSCO, or any other photo editing application, then you're already familiar with using presets. Presets are Lightroom's version of filters. Choose a preset you want to apply to a photo, click it, and voila!
What makes Lightroom presets so powerful is how comprehensive they can be. While apps like Instagram offer 10-15 different adjustments that you can apply to a photo, Lightroom has over 100 different available adjustments. Presets, in turn, can include any combination of those adjustments to create a nearly infinite number of unique looks.
Interested in testing out the same presets I personally use to edit photos? Look no further.