Psalm 119:11-12
11 I have hidden your word in my heart
that I might not sin against you.
12 Praise be to you, LORD;
teach me your decrees.
Remember the psalmist is answering the question of how a young man can keep his way pure. His answer is to live according to God's word. The rest of this section is describing how one can live according to the word of God.
The word translated hidden can also be translated store up. It carries the connotation of hiding, storing something of great value. Treasure, as a verb, is another good translation of this word (Pr. 2:1).
The word praise is barak which carries the connotation of thanking the LORD for his blessings. This blessing the psalmist has in mind is the decrees of the LORD.
Word ('imrāh) in v. 11 can also carry the specific connotation of God's promises (Ps. 119:50, 58, 76). If we read it with that connotation the psalmist is saying that it is the promises of God that he treasures in his heart that keep him from sinning. God's covenant promise of faithfulness to Israel, his promise of deliverance through the Messiah, his promise of just punishment for disobedience and blessing for obedience are all treasured in the heart of the psalmist to keep him from sinning.
In the Message, Eugene Peterson writes, "I've banked your promises in the vault of my heart so I won't sin myself bankrupt."
I've always read this verse as an incentive to memorize specific Scriptures to help me do spiritual battle to defeat temptation as Jesus did in the wilderness (Matt. 4:1-11; Eph. 6:17). If it is to be read with the promise connotation of this word then the meaning is slightly different than that. It is more of an awe and wonder at the covenant faithfulness of God and his commitment to fulfill all that he has promised that keeps the psalmist from sin. It is more of a, "God is beyond good, gracious, and faithful to me, how can I sin against him?" Or it is a, "Remember the blessing that God has promised for obedience to his law, don't throw it away for this sin."
To be clear, this word can also carry the broader meaning of just word as well (Ps. 119:67, 162). The last couple of paragraphs are just kinda my process of exploration and study with this word.
The word decrees implies an official kingly decree or rule. A law passed through the judiciary and now in effect is how we would think of it today.
Reflection
Whether the meaning of the word 'imrāh is broader and refers to word or more specific referring to promise, we need to 1. know God's promises and his word by engaging with it regularly and faithfully, 2. cherish it, bank it as a precious treasure. Think of a promise of God and spend some time reflecting on it, considering how the goodness of that promise can keep you from sin.