What does Psalm 103:20 mean?

In this verse David calls on angels to bless, which in this context means to praise, their God. He describes the angels as mighty and obedient to God's Word. They listen for the faintest disclosure of His will.

The adjective "mighty" means powerful in strength and valor, and it is applied to warriors or conquerors or men of great ability (Judges 6:12; 11:1; 1 Chronicles 26:6). The account of the Assyrian forces that laid siege to Jerusalem in the time of King Hezekiah shows how mighty the angels are. The angel of the Lord—a single being—descended on the Assyrians during the night and killed 185,00 of them (2 Kings 19:35). While some think that might have been a pre-incarnate Jesus, any being described as an "angel" in the Bible bears immense power.

When Jesus returns to earth, He will be accompanied by His mighty angels and will judge those who do not know God and do not obey the gospel (2 Thessalonians 1:7–8). In the tribulation period a mighty angel with a loud voice, like a lion roaring, will stand with his right foot on the sea and his left foot on the earth (Revelation 10:2–3).