Diocese and Archdiocese
Landscape of the church
Jesus is the head of the church. He sits at the right hand of our Father who art in heaven. From his rightful place he reigns forever. This Ascension requires all of us to be his representatives on earth.
The Pope is the head of the church on earth. He reigns from Rome as long as he lives. Being responsible for the church throughout the world, he sends the disciples of Jesus to every land and every nation. Each one is the Bishop of the land he his sent to and called to serve.
The land he is sent to is called a diocese. The bishop of the diocese has the ordinary responsibility to govern the church on the land where he serves. Church law gives him the full authority to govern that diocese.
By the command of Jesus, the church grows. As the populations of Catholics grow or an area becomes more significant it could make sense to create a new diocese to more effectively govern the area. In such a situation, the Pope may be or has been wise to name a particular diocese an archdiocese.
This gives the bishop of that land the title Archbishop and some responsibility to assist the bishop of dioceses surrounding his archdiocese. Outside of his diocese, he would have limited authority, but great influence.
All of this organization is to help govern the church and keep as all near the Lord so that he, Jesus, the head of the church is well represented and available at any time in all spaces.