When you make an inroad, you advance into new territory. An attacking army might try to make an inroad into a neighboring country, for example.
Intruding in another's space or advancing into a previously inaccessible area are both examples of inroads. The word is most often used in its plural form, like when your history teacher talks about the great inroads women have made politically in the past fifty years. The noun inroad was originally used only to talk about military movement, describing armies making hostile raids. The road part of inroad evolved from the word ride, so the word comes from "riding in."