The Catechism in Questions & Answers

459. What are the duties of the Chief Apostle?

The Chief Apostle maintains the unity among the Apostles. He strengthens the Apostles (cf. Luke 22: 32), and “tends” the flock of Christ (cf. John 21: 15-17). He ensures that the gospel is proclaimed in unadulterated fashion. Through the Holy Spirit, he opens up new insights and interconnections in the doctrine of the Church and is responsible for its uniform spread. Beyond that, he establishes the order within the Church.
All of these duties comprise the “authority of the keys” of the Chief Apostle ministry.
The Chief Apostle ordains the Apostles. Together with them, he leads the Church.

“I do not pray for these alone, but also for those who will believe in Me through their word; that they all may be one.” John 17: 20, 21
“And I will give you the keys of the kingdom of heaven, and whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.” Matthew 16: 19