Rashi on Leviticus 14:4/רש"י על ויקרא י״ד:ד׳

1חיות LIVING [BIRDS] — This term excludes רפות‎ט, birds that suffer from some fatal organic disease (חיות does not only mean “living”, but also “capable of continuing to live”).

2הרות‎ט CLEAN — This term excludes an unclean bird (of a species that may not be eaten) (cf. Chullin 140a). Because the plagues of leprosy come as a punishment for slander, which is done by chattering, therefore birds are compulsory for his (the leper’s) purification, because these chatter, as it were, continuously with a twittering sound (Arakhin 16b).

3ועץ ארז AND CEDAR WOOD — This lofty tree was used because plagues come also as a punishment for haughtiness (cf. Arakhin 16a).

4ושני תולעת ואזוב AND CRIMSON AND HYSSOP — What is the remedy he should use, that he may be healed? Let him, abandoning pride, regard himself lowly as a worm (תולעת) and as hyssop (cf. Midrash Tanchuma, Metzora 3).

5עץ ארז means a stick of cedar wood (not a cedar tree).

6שני תולעת — a strip of wool dyed crimson.