What does Numbers 3:41 mean?

He said, 'Set the Levites apart for me instead of all the firstborn sons of Israel. Take the Levites' animals instead of the firstborn animals from the rest of the people. The Levites belong to me.' Numbers 3:41 - Modern Text Bible

(God claimed the Levites and their animals as substitutes for the firstborn sons and animals of all Israel, marking them as his own.)