It’s been three months since the Israelites left Egypt. They are now in the wilderness of Sinai by Mt. Horeb. The Lord told Moses he rescued them to set apart the people of Israel and make them a kingdom of priests and a holy nation. He told them to wash their clothes and not sleep together to get ready for the third day. 

On the third day, the presence of the Lord came with thunder and lightning, the mountain shook and smoke wrapped around the mountain. The people could get close but not touch it. God told Moses to have Aaron join him up the mountain because he was going to tell them something important. 

Here the people thought God was rescuing them out of Egypt to go find a paradise to hang out in. God has a bigger plan for them, they will be his priests, the ones who represent him to the world. They have to be clean and disciplined in order to be able to receive their assignment.