The Israelites will worship the Lord on this mountain after the exodus. A sign is meant to assure the people that God’s message is trustworthy. This is a long-term sign. In Exodus 4:7–8 God gives signs to Moses and the Israelites for the short term. The Israelites will worship the Lord. The word in Hebrew for worship is serve.
Now the Israelites serve (= do work) the Pharaoh in slavery, but they will be liberated to serve (= worship) the Lord.
12 He said, “But I will be with you, and this shall be the sign for you, that I have sent you: when you have brought the people out of Egypt, you shall serve God on this mountain.”