And it shall be for a sign and for a witness unto the LORD of hosts in the land of Egypt: for they shall cry unto the LORD because of the oppressors, and he shall send them a saviour, and a great one, and he shall deliver them.
This means that in Egypt, people will use this as a sign and proof to show their belief in God. They will call out to God because they are being treated unfairly. God will then send them a powerful rescuer who will save them from their problems.