For all have sinned is self explanitory, fall short of the glory of God I think means that we cannot fully understand all of who He is.
Man, do I relate with this! It is one of my wonderful struggles over the years. Haha! Actually, I really do look at struggles as wonderful. They don't feel wonderful, but they are. There are such riches buried just below the crusted surface. (If you didn't follow that, don't worry, I'm not too right in the head!! It's just the coolness of realizing that God's opinion is the correct one and that just because I think things aren't right at the present doesn't mean that's really what's going on right now.)
There is a verse, that to me, relates, in 1 Corinthians that goes like this:
but just as it is written, 'Things which eye has not seen and ear has not heard, and which have not entered the heart of man, all that God has prepared for those who love Him.' 1 Corinthians 2:9
When you put it together with the following...
The secret things belong to the Lord our God, but the THINGS REVEALED belong to us and to our sons forever, that we may observe all the words of this law Deuteronomy 29:28
...you get the concept that is taught in most Christian churches that we can only know the things of God that He had revealed in the written word.
But something about this doesn't sit well. Why would did Paul even mention this verse if he was only rehashing a basic premise of our limits of understanding God? So, this is exactly what the statement above refers to. Actually, falling short of the glory of God is the reason that the closest man could understand God was through doctrines (recorded statements and the teachings based on those statements). The problem with this understanding of God is that it is not true knowledge of God ... only of FACTS about God. Falling short is why man NEVER understood God at all.
This is why Paul quoted an Old Testament verse about the limitations of knowing the things of God so that he could then say,
For to US God revealed them through the Spirit; for the Spirit searches all things, even the depths of God. 1 Corinthians 2:10
We have been given so much more than a whole new bunch of teachings about God (which is all they had been given in the Law) ... we have actually been given the mind of Christ, to think God's thoughts. This is the miracle! Delivered from the realm of darkness and brought into the glory of God. So cool!