וְאֶל־הַלְוִיִּ֣ם תְּדַבֵּר֮ וְאָמַרְתָּ֣ אֲלֵהֶם֒ כִּֽי־תִ֠קְחוּ מֵאֵ֨ת בְּנֵֽי־יִשְׂרָאֵ֜ל אֶת־הַֽמַּעֲשֵׂ֗ר אֲשֶׁ֨ר נָתַ֧תִּי לָכֶ֛ם מֵאִתָּ֖ם בְּנַחֲלַתְכֶ֑ם וַהֲרֵמֹתֶ֤ם מִמֶּ֙נּוּ֙ תְּרוּמַ֣ת יְהוָ֔ה מַעֲשֵׂ֖ר מִן־הַֽמַּעֲשֵֽׂר׃
"Parle aussi aux Lévites et dis-leur: Lorsque vous aurez reçu des enfants d’Israël la dîme que je vous donne de leur part, pour votre héritage, vous prélèverez là-dessus, comme impôt de l’Éternel, la dîme de la dîme.
Sefer HaMitzvot
That is that He commanded the Levites to extract a tithe from the tithe that they take from Israel, and give it to the priests. And that is His saying, "Speak to the Levites and say to them, 'When you receive the tithes from the Children of Israel, etc.'" (Numbers 18:26) And Scripture explains that this is called the priestly tithe from the tithe (terumat maaser) and is given to a priest. And Scripture explains that this tithe is taken either from the goodly [part] of it or from the fine part of it, by its saying, "from any of its best portion (chelbo)" (Leviticus 18:29). And that is His, may He be blessed, saying, "And you shall bear no sin by reason of it, when you have set aside from it the best of it" (Leviticus 18:32).
Sefer Hamitzvot of Rasag
The Levites must donate from their tithe to those [priests] who serve the Ruler inside.