This verse, from the Song of the Three Holy Children (a deuterocanonical book in the Bible), is a call to praise and thanksgiving directed towards all who worship God. Let's break it down:

"O all you that worship the Lord...": This is an inclusive invitation to everyone who acknowledges and reveres God.
"...bless the God of gods...": To "bless" God doesn't mean to bestow a benefit upon Him, but rather to acknowledge His goodness, power, and majesty, and to speak well of Him. The phrase "God of gods" emphasizes God's supreme authority and power over all other deities or beings.
"...praise him...": This is a direct call to offer enthusiastic expressions of adoration and admiration to God.
"...and give him thanks...": This encourages gratitude for God's blessings and providential care.
"...for his mercy endures forever.": This is the key reason for the call to worship. It highlights the enduring nature of God's loving-kindness, compassion, and forgiveness. God's mercy is not fleeting but is a constant attribute that continues through all time.

In essence, the verse is a powerful affirmation of God's greatness and goodness, urging all believers to respond with heartfelt praise, thanksgiving, and recognition of His eternal mercy.