Barak is the Hebrew verb to bless, and it carries a beautiful double sense. Its root is connected to kneeling — so barak can describe God blessing us (bending down to do us good) and us blessing God (kneeling in worship and praise). The same word covers both directions of the relationship.
So when the psalmist says Bless the LORD, O my soul, he is calling himself to kneel in grateful worship; and when the priest pronounces the LORD bless thee and keep thee, God is bending toward his people in favour. To barak God is not to add to him but to bow before him with thanks. Blessing, at its heart, is a posture of the knee and the heart alike.
Barak in Scripture
Bless the LORD, O my soul: and all that is within me, bless his holy name.