כָּל־פֶּ֣טֶר רֶ֠חֶם לְֽכָל־בָּשָׂ֞ר אֲשֶׁר־יַקְרִ֧יבוּ לַֽיהוָ֛ה בָּאָדָ֥ם וּבַבְּהֵמָ֖ה יִֽהְיֶה־לָּ֑ךְ אַ֣ךְ ׀ פָּדֹ֣ה תִפְדֶּ֗ה אֵ֚ת בְּכ֣וֹר הָֽאָדָ֔ם וְאֵ֛ת בְּכֽוֹר־הַבְּהֵמָ֥ה הַטְּמֵאָ֖ה תִּפְדֶּֽה׃
Every thing that openeth the womb, of all flesh which they offer unto the LORD, both of man and beast, shall be thine; howbeit the first-born of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.
Read Kabbalistic (mystical) interpretations of Numbers 18:15: Zohar and classical Jewish mystical sources, free online.