What it means to fall short of the glory of god?

We experience the glory of God as we walk in fellowship with Him.

I John 1:7 "But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin."

Because none of us walk perfectly, we need a savior.

Romans 3:23 "For all have sinned, and come short of the glory of God;"

All men except Jesus Christ have broken fellowship with God, which is sin. Fulfilling all the Old Testament law, Jesus:

Hebrews 4:15 "...was in all points tempted like as we are, yet without sin."