The Church’s General Handbook explains callings in The Church of Jesus Christ of Latter-day Saints:

Heavenly Father gave Jesus Christ a sacred mission to fulfill (see Luke 4:18–19; John 6:38; 3 Nephi 27:14–16). During His ministry, the Savior trusted His disciples with important responsibilities (see Luke 10:1–9). Likewise, the Lord calls men and women to serve in the Church today through inspired invitations from His servants. These opportunities to serve are known as callings.
 
Callings give members opportunities to feel the joy of serving God by serving His children (see Mosiah 2:17). Callings also help members increase their faith and draw closer to the Lord.
 
It is not appropriate to aspire to specific callings in the Church (see Mark 10:42–45; Doctrine and Covenants 121:34–37). Nor do Church members “advance” from one calling to another. After serving in one calling, they should not expect to receive a calling of greater responsibility or prominence. Serving faithfully in a calling is more important than what the calling is. The Lord honors the dedication of all who serve in His Church.

To learn more about how members are called to serve in the Church, see the General Handbook, 30.8.