In his poem entitled "Shakespeare," Matthew Arnold praises the great dramatist and poet William Shakespeare.  

Arnold praises Shakespeare's extraordinary knowledge of human nature:

[Your knowledge is] out-topping knowledge.

Shakespeare's greatness is divine:  "The heaven of heavens [is] his dwelling place"; "the stars and sunbeams [you] know."

On the other hand, Arnold praises Shakespeare for being down-to-earth, for being able to communicate with simple people, and for being able to appear to most people as being nothing extraordinary:

Thou...Didst tread on earth unguessed at.

The poet comments on this with the expression, "Better so!"  He seems to mean that it was better that Shakespeare was, on the surface, just like anyone else.  In this way, he was able to mingle with regular human beings and learn about their nature.  Because he was "Self-schooled, self-scann'd," he was able to learn much more about human nature than a scholar who did not have much opportunity to observe life in action.