“备胎”，本意是指“给汽车准备的备用轮胎（spare tire）”；如今，在网络用语中多指一段感情中的替补、备选人，与英语流行语“back-burner guy/girl”意思相近，表示“a boy or girl of slight interest who isn't exactly qualified to date at the moment, but may come in handy later”。
She’s your back-burner girl, eh?
I don't really like her. She's just my back-burner girl. This rebound relationship doesn't count.