Tags are labels you add to your WP posts to keep stuff organized (better SEO overall).
When you're writing a post, there's a "tags" box on the right side, just add the word that makes sense for that post and hit add. Simple as that.
Try to be specific, do not overuse it, for example, if you article is explaining how to make some good food, tags should be "food" or "recipe" etc, nothing big, one or two words.
Think about what your readers might search for, of course, if you decide to let Google index tags which some people don't do, like me for example, at first I did but later, nope, just categories, but in general it depends from niche to niche.
Just don't spam with tags, don't use tags as your main keywords, keep it simple, and use tags in a way that makes sense for you and your readers.