The implication is typically that they’re popular with guys. ie, they’re physically attractive to the opposite sex, they are sex symbols. That’s really what it’s all about. It’s not a two-way label, it doesn’t mean the attraction is necessarily returned. Although it is often assumed and commonly leads to people accusing them of being “sluts” despite not being justified. But on its own it just means that they are the object of desire of many boys: they probably have many suitors, lots of people want to ask them to the dance, etc. That is the way they are considered “popular”.