What does Hebrews 12:21 mean?

One of the great heroes of faith mentioned in Hebrews chapter 11 was Moses. It was Moses whom God called onto Mount Sinai, after the exodus from Egypt, in order to receive the old covenant (Exodus 19:9–20). As explained in earlier verses, this involved a magnificent display by God, but this display was also terrifying. The combination of smoke, fire, loud sounds, and a command to not even touch the mountain was intimidating even to Moses (Deuteronomy 9:19). That, as it turns out, was part of the point of the entire incident. Like the old covenant itself, God's introduction of His law was meant to foreshadow a better, more complete arrangement later on: the new covenant in Jesus Christ.

The mention of Moses here helps to tie some of these different ideas together. Moses was already commended for his faith (Hebrews 11:24–28), and now he's included in the list of those who were frightened by God's presentation of the old covenant. This highlights the fact that God's awesome display was meant to inspire a certain kind of fear, and also to provide contrast to the eventual ministry of Christ. As the next verses point out, the new covenant is "presented" to mankind in a much more inviting, peaceful manner.