Continuing the account of when God gave Israel the ten commandments, the people are afraid. And in reality when anyone stands before God there should be an element of fear. Bit I think this fear was from a couple of reasons, they hear a voice and can't see it's source so there is this fear of the unknown. And I also think that as God lists off each commandment they become convicted when their own sins are named. Israel received this law before Moses was instructed in making the tabernacle and the offering system, there was at that time no means to ask forgiveness. That would be very scary.