Catalog
The catalog is the database file in which Lightroom Classic keeps track of everything: where your photos are, your edits, keywords, stars and collections. The photos themselves aren't in it, the catalog only points to them. One catalog can manage thousands of photos.
Important to understand: editing in Lightroom Classic doesn't change your RAW file, everything is stored in the catalog. So back up your catalog regularly, separately from your photos. Always move your photos within Lightroom (not in Explorer or Finder), otherwise the link is lost. Tip: let Lightroom back up the catalog automatically when you quit.
Prefer a look in one click?
The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.