I don't know the meaning of "in"

[Unregistered]

Overdoing it and improper diet often play a role.
For others, genetic factors or allergies factor in

[RonBee]

Overdoing it and improper diet often play a role.
For others, genetic factors or allergies factor in

It isn't the word "in" that is relevant here. The term "actor in" is a phrasal verb which means "play a part in" or "have to do with (something)" or "play a role in (something)" or "have a relevant role in". Genetic factors and allergies are an important part of whatever it is.