Psalm 19:14

Matthew 12:36 - Every idle word you speak, you will give an account for

We all have thoughts and conversations with ourselves going on in our head. They can be:

Good thoughts: I Samuel 30 - But David encouraged himself in the Lord.
Bad thoughts: 2 Kings 5:20 - Elisha instructs Naaman to dip in the Jordan River and is healed from his leprosy. Elisha refuses payment from Naaman; Gehazi goes to Naaman and accepts the reward with a lie to God and to people.

Meditations of the heart are very important.
Proverbs 4:23 - Keep thy heart with all diligence.

David prays in this Psalm for accountability with the thoughts of his heart.