Bishops are recognized as people who have the Holy Spirit inside of them. In the early church they presided over Sunday liturgy, preached, baptized and forgave sins. Another Role of bishops was the episcopal synods, which where forums where bishops would discuss problems and how to solve them within the church.
The Second Vatican Council reemphasized the bishop's Role of preaching and teaching while adding a new role: they were each in charge of their own diocese, not just watching over it for the pope. This created a feel of a college of bishops, working together to fulfill christ's mission. This can be called the doctrine of episcopal collegiality.