What is a coadjutor?
A coadjutor bishop appointed to a Catholic diocese or archdiocese to assist the diocesan bishop. Unlike an auxiliary bishop, he has the right of succession, meaning that he automatically becomes the new bishop when the diocesan bishop retires or dies. By canon law, he is also vicar general of the diocese. If the diocese is an archdiocese, he is called coadjutor archbishop instead of coadjutor bishop.
In recent years, a growing number of U.S. bishops in larger dioceses or archdioceses have requested and received a coadjutor in their final year or two before their retirement, in order to familiarize their successor with the workings of the (arch)diocese before he has to take over the reins. This minimizes the learning curve of a new bishop and eliminates completely the possibility of the diocese being vacant following the old bishop’s retirement.
From https://www.usccb.org/offices/general-secretariat/appointing-bishops
and Code of Canon Law – Coadjutor bishop