What does Proverbs 15:11 mean?

Following up on his prior statement (Proverbs 15:10), Solomon explains how God's infinite knowledge includes understanding death, eternity, and the consequences of sin. Sheol is the word for the unseen world, used in the Old Testament to refer to death or the grave. In other words, the Lord sees everything and everyone in the realm of the dead.

God also sees Abaddon and everything in it. Abaddon, meaning "destruction," corresponds to what the New Testament sometimes calls "the abyss:" the dwelling place of evil spirits or departed souls (Romans 10:7; Revelation 9:1–2). Luke 8:31 reports that the demons that Jesus cast out of a man in the country of the Gerasenes begged Jesus not to send them into the abyss. Revelation 9:1 and 11 refer to the abyss as "the bottomless pit," and names the king of demons as Abaddon.

Surely, if Sheol and Abaddon are visible to God, He can see the hearts of human beings (Hebrews 4:13)! This openness of the heart relates to Jesus' knowledge of the human heart. John 2:25 says Jesus "needed no one to bear witness about man, for he himself knew what was in man."