What Are Tags?
Starting in macOS 10.9 Mavericks, files could be given one or more "tags", to help users keep things organized. A tag is simply a text label you can assign to a file, letting you organize your files with categories instead of, or in addition to, folders. For example, a student might have a science folder, holding a textbook, handouts, tests, notes, assignments, research papers, and more. What if a quiz deals with thermodynamics, as do some files of notes and a paper. Do you put them into folders according to subject, or according to what sort of document they are (quiz, notes, paper)? With tags, you can have both. Just tag all the thermodynamics files with a "thermodynamics" tag, the notes with a "notes" tag, and so on. Now you can find all your notes, all your thermodynamics notes, all your work related to thermodynamics… You get the idea.