What does Psalm 103:21 mean?

David addresses all the Lord's hosts in this verse and declares they are the Lord's "ministers" who do His will. This is from the Hebrew term mesor'tāy, which describes the action of a servant. This comes from the root Hebrew word sharath. The term "hosts" may apply to the angels mentioned in the previous verse. Psalm 103:20 refers to the angels as mighty, whereas this text refers to them as a "host." Scripture does not give an exact number of angels, but this does not mean there are an infinite number of them. However, there seem to far more of them than a human being could count. Hebrews 12:22 mentions innumerable angels in festal gathering.

When Jesus was born, an angel announced His birth to a group of shepherds. Luke 2:13 states: "And suddenly there was with the angel a multitude of the heavenly host praising God." Isaiah 6:1–3 and Revelation 4:8 give us a glimpse of heavenly beings near God's throne. They praise Him and exclaim, "Holy, holy, holy, is the Lord God Almighty, who was and is and is to come!" (Revelation 4:8). Some Bible teachers include the heavenly bodies themselves in the hosts that David calls on to praise the Lord.