When I was in school, being a "popular kid" meant that someone was liked or admired by a lot of people. They were high profile, usually on a sports team or cheerleading/drill team. A lot (though not all) of the "popular kids" were really smart. They were very active in school activities. They won homecoming queen and prom queen and class officer positions. They were just . . . popular.

I was never one of the "popular kids." I was smart, and I was in some school activities, but I wan't in demand for parties, no one asked me to prom or other school dances, etc.