我深知我是一个备胎
[wŏ shēn zhī wŏ shì yī gè bèi tāi]
'I Clearly Know I'm Just A Spare Tire' is metaphorical language. People often say that if a girl/guy likes someone who already has a boyfriend/girlfriend but can't stop missing them, they choose to silently support and accompany this person from side like the spare tire of a bike/car, waiting for an unknown future