שחין. לְשׁוֹן חִמּוּם, שֶׁנִּתְחַמֵּם הַבָּשָֹר בְּלִקּוּי הַבָּא לוֹ מֵחֲמַת מַכָּה שֶׁלֹּא מֵחֲמַת הָאוּר (חולין ח'): שחין A BOIL — The word expresses the idea of heat. It is so called because the flesh has become inflamed by an injury which came about as the result of a blow and not as the result of fire (the latter case being mentioned in v. 24 and being termed a מכוה) (Chullin 8a).
ונרפא. הַשְּׁחִין הֶעֱלָה אֲרוּכָה וּבִמְקוֹמוֹ הֶעֱלָה נֶגַע אַחֵר: ונרפא AND IT BECAME HEALED — the boil (not the flesh also mentioned in this verse) became healed and in its place it produced another plague (the שאת mentioned in the next verse).