D&C 13

With the restoration of the Aaronic Priesthood, John the Baptist taught -

"...the Priesthood of Aaron, which holds the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins..." (Doctrine and Covenants Section 13:1)

It is these initial keys the president of the Church of Jesus Christ of Latter-day Saints possesses. It is under his direction that bishops preside and are are given these key over jurisdiction boundaries for -

"The bishopric is the presidency of this priesthood, and holds the keys or authority of the same." (Doctrine and Covenants Section 107:15)

This means that bishops by virtue of the president of the priesthood hold three keys -

1) the keys of the ministering of angels
2) the keys of the gospel of repentance
3) the keys of baptism

The definition of angels in my mind is a messenger sent from God. Angels then could be spiritual beings, glorified beings, and even earthly messengers. The priesthood is authority to act for God. It is the bishop that holds keys to the "ministering" of angels. This may include the help of guardian angels as the bishopric prays on behalf of ward members. In addition, the bishopric holds keys to oversee and organize the ward council, home teaching, visiting teaching, and other forms of "ministering" angels to minister to ward members.