What does Numbers 18:26 mean?

'Speak to the Levites and tell them: When you receive the tithes from the Israelites, which I've given you as your share, you must give a tenth of those tithes as an offering to God.' Numbers 18:26 - Modern Text Bible

(Even the Levites, who receive tithes, are required to give a tithe of what they receive back to God, showing that everyone is accountable to him.)