What does Numbers 18:17 mean?

But you can't redeem the firstborn of cattle, sheep, or goats. They're holy and must be sacrificed. Sprinkle their blood on the altar and burn their fat as an offering with a pleasing smell to the Lord. Numbers 18:17 - Modern Text Bible

(Firstborn animals that can be sacrificed must be offered to God, not redeemed. Their blood and fat are part of the ritual.)