What did Jesus mean in Matthew 23:13-15?

What is meant by it (whether or not it's accurate is another question) is that the "scribes and Pharisees" knew enough to be able to control lots of others, and by their teachings and pronouncements, they barred the way to the "kingdom of heaven." Those who know enough to make things miserable for others and are powerful enough to carry this out "will receive greater condemnation." And (in v. 15), he is saying that following the ways of the "scribes and Pharisees" only leads to and other misery.