Hebrew kodesh hakodashim — "the holy of holies, the most holy place." The innermost chamber of the tabernacle and (later) the temple, separated from the outer Holy Place by the veil. It contained the Ark of the Covenant (in the tabernacle and Solomon's temple; the ark was lost before the Second Temple, leaving the chamber empty in Herod's day). The Holy of Holies was a perfect cube — 10 cubits on each side in the tabernacle, 20 cubits in Solomon's temple — symbolizing perfection and completion. Only one man entered it only one day a year: the high priest on the Day of Atonement (Yom Kippur), and only after elaborate purification.
The Holy of Holies was the architectural expression of the core problem of theology: how can sinful humanity dwell with a holy God? The chamber was the earthly echo of God's throne-room presence — "I will meet with you there, and from above the mercy seat, from between the two cherubim that are on the ark of the testimony, I will speak with you" (Exodus 25:22). Elaborate separations and purifications kept the unclean out; only blood-bearing priesthood entered. This speaks. Then — at the moment of Jesus' death on the cross — "the curtain of the temple was torn in two, from top to bottom" (Matthew 27:51). Not bottom to top (as if man tore it). Top to bottom — from God's side. The Holy of Holies was opened. Hebrews 10:19-22 expounds the implication: "Since we have confidence to enter the holy places by the blood of Jesus, by the new and living way that He opened for us through the curtain, that is, through His flesh... let us draw near with a true heart in full assurance of faith." Every Christian at prayer has access the OT high priest had only once a year. The Holy of Holies is now open. Walk in.