The goal of your title/cover is to get someone to read your synopsis, and the goal of your synopsis is to get someone to click on your first chapter.
1. Get to the point quickly. Do not pack your synopsis with paragraphs of detailed exposition.
2. Give a general overview of the MC's motivation, goal, and/or predicament, so the reader knows what to expect.
3. Be careful about using names for people/places/organizations/etc., and only use like two or three. Throwing around made-up titles or the names of kingdoms is useless and distracting, so keep stuff like that to a minimum. The MC's name alone is often good enough for the synopsis, with maybe one or two particularly important names alongside that.
4. Your goal is not to be mysterious, confusing, or to trick the reader. Your goal should be to find those who are interested in what you are offering and are willing to read it, otherwise anyone you hook will leave once they realize the story isn't what they expected.
5. Have good grammar! If I see that an author hasn't even taken the time to fix the errors in their own synopsis, I automatically assume that their chapters will be just as bad if not worse. It just makes you look bad, is the main issue. I can read stories without perfect grammar, but at least put in a bit of effort!