五百二十
[wŭ băi èr shí]
Quite literally, it means '520'. While seeming meaningless at first glance, this combination represents a homophone for 'I Love You', given how 5 (Wu), 2(Er) sounds like 我(Wo-I), 爱(Ai-love), 你(Ni-you). A rather cute way of showing affection!