Kataskiázō (κατασκιάζω) means to overshadow, to cast a shadow over, to cover with shade. From kata (down upon) + skiazō (to shade). It appears once in the NT in the description of the tabernacle.
Hebrews 9:5 describes the cherubim of glory 'overshadowing the mercy seat.' The mercy seat (hilastērion) was where atonement was made — where God's presence met the blood of sacrifice. The cherubim overshadowing it represented divine guardianship of the place of reconciliation. This imagery runs from Eden (cherubim guarding the way to the tree of life, Genesis 3:24) through the tabernacle to Christ Himself, who is our mercy seat (Romans 3:25). The shadow of the cherubim speaks of holy protection over the place where God and sinners meet.