ELI5: why is mercury’s orbit different than the other planets in our solar system?

is it bc of its distance between the sun or about the mass of mercury? i barely know anything about math and especially whatever subject this is considered LOL! and what does relativity have to do with mercury’s orbit since i read that it had something to do with helping to prove einsteins theory???? relativity helps predict mercury’s orbit, how??? im sorry, i hope what im asking isnt really stupid lol im just wondering hehe