"Jelly" is another word for a jellyfish. It is also a slang (short language) for "jealous". Yes, don't be jealous! You'll never be or beat what you jealous.
Control yourself and stop feeling envious of someone's achievements.
Rather than being jealous, truthfully celebrate the person.
You gain more from celebrating than feeling resentments towards someone else.
Be the bigger person. There's no good feeling bad for yourself because of someone's possession
You've got your own life, live it. Mind your business.