The typology of the High Priest entering the Holy of Holies is rich in meaning. Only he was the mediator directly between God and His people. A sinful person could not enter into this room, because he would die in the very presence of God.  So something had to be first done about the sin problem before man could enter in.  The High Priest could enter in only if he obeyed God in observing all the rituals of cleansing so that he would exactly picture the Sinless One Who would one day come as man's Eternal High Priest.  Because the blood of bulls and goats only dealt with sin temporarily and did not remove sin for all time and in every way, the common man could not, nor even the common Levitical priests could not, ever enter into the Holy of Holies. But when Christ came as the Eternal High Priest, this way was opened. This is explained in Hebrews 9:1-15:

Then verily the first [covenant] had also ordinances of divine service, and a worldly sanctuary.

For there was a tabernacle made; the first, wherein [was] the candlestick, and the table, and the shewbread; which is called the sanctuary.

And after the second veil, the tabernacle which is called the Holiest of all;

Which had the golden censer, and the ark of the covenant overlaid round about with gold, wherein [was] the golden pot that had manna, and Aaron's rod that budded, and the tables of the covenant;

And over it the cherubims of glory shadowing the mercy seat; of which we cannot now speak particularly.

Now when these things were thus ordained, the priests went always into the first tabernacle, accomplishing the service [of God].

But into the second [went] the high priest alone once every year, not without blood, which he offered for himself, and [for] the errors of the people:

The Holy Ghost this signifying, that the way into the holiest of all was not yet made manifest, while as the first tabernacle was yet standing:

Which [was] a figure for the time then present, in which were offered both gifts and sacrifices, that could not make him that did the service perfect, as pertaining to the conscience;

[Which stood] only in meats and drinks, and divers washings, and carnal ordinances, imposed [on them] until the time of reformation.

But Christ being come an high priest of good things to come, by a greater and more perfect tabernacle, not made with hands, that is to say, not of this building;

Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption [for us].

For if the blood of bulls and of goats, and the ashes of an heifer sprinkling the unclean, sanctifieth to the purifying of the flesh:

How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?

And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions [that were] under the first testament, they which are called might receive the promise of eternal inheritance

The veil in a previous study, was shown to be the separation between the Holy Place and the Holy of Holies, blocking and barring those who would attempt to enter in before the throne of God.  Once in the Holy of Holies, the High Priest stood in the very presence of God, carrying out the atoning ritual for the people of God, before the Holy Cherubim, before the Mercy Seat, and the Ark which held the budding rod, the tablets of law, and the pot of manna. More will be said on the Day of Atonement later.  It was a very special day when all of the past year's sin committed by all of the tribes of Israel was atoned for. Christ is our High Priest, our Day of Atonement, and our Perfect Sacrifice. It is only by His shed Blood at Calvary that the eternal atonement takes place.

The Holy of Holies was a perfect square, 15 feet by 15 feet by 15 feet.  This speaks of perfection and Holiness.

As stated, only the High Priest on one day each year was allowed to enter into the Holy of Holies.  But when Jesus Christ became our High Priest, the veil was rent that stands between the two rooms and essentially both rooms become one because of the rent veil.  Now the believer, ushered in by his High Priest, Jesus Christ, may come "boldly before the throne of grace," united with the Father by the Son.  Hebrews 10:19-22

Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus,

By a new and living way, which he hath consecrated for us, through the veil, that is to say, his flesh;

And [having] an high priest over the house of God;

Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.