What is the "joy of the Lord" in Nehemiah 8:10?

 Ne 8:10 Then he said to them, "Go your way, eat the fat, drink the sweet, and send portions to those for whom nothing is prepared; for this day is holy to our LORD. Do not sorrow, for the joy of the LORD is your strength."

Just move down to verse 12 and you will see the parallel:

Ne 8:12 And all the people went their way to eat and drink, to send portions and rejoice greatly, because they understood the words that were declared to them.

It refers to the joy of understanding God's will. Joy is a great encourager. When you delight in God's will you will do it.