# in the first month of the second year after they came out from the land of Egypt

This means that they had come out of Egypt a year earlier. They were beginning their second year in the wilderness. 

# in the first month

This is the first month of the Hebrew calendar. It marks when God brought the people of Israel out of Egypt. 

# after they came out from the land of Egypt

"after they left the land of Egypt". Here "they" refers to the people of Israel.