"Birth order" refers to whether a child is the oldest, second-oldest, youngest, etc. in their family. For a while, pop psychologists created a whole industry around telling people how their birth order affected their personality: oldest children are more conservative, youngest children are more creative, etc.

Then people got around to actually studying it and couldn't find any of that. Wikipedia's birth order article says:

Claims that birth order affects human psychology are prevalent in family literature, but studies find such effects to be vanishingly small….the largest multi-study research suggests zero or near-zero effects. Birth-order theory has the characteristics of a zombie theory, as despite disconfirmation, it continues to have a strong presence in pop psychology and popular culture.