Psalm 103 vs. 1-5 says "Bless the LORD, O my soul and all that is within me bless His holy name. Bless the LORD, O my soul and forget not His benefits: Who forgives all your iniquities, Who heals all your diseases, Who redeems your life from destruction, Who crowns you with loving kindness and tender mercies, Who satisfies your mouth with good things, so that your youth is renewed like the eagle's."
Psalm 68 vs.19 says Blessed be The LORD, Who daily loads us with benefits, The God of Our salvation! Selah.
To bless the LORD in very simple terms means to praise, thank and appreciate God with all honour for His loving kindness and tender mercies towards us.