herb vs healing herb

herb and healing herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
herb	Yes	No	No	No
healing herb	Yes	No	No	No

As nouns, healing herb is a hyponym of herb; that is, healing herb is a word with a more specific, narrower meaning than herb:

herb: aromatic potherb used in cookery for its savory qualities
healing herb: leaves make a popular tisane; young leaves used in salads or cooked

herb (noun)	healing herb (noun)
aromatic potherb used in cookery for its savory qualities	leaves make a popular tisane; young leaves used in salads or cooked
a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests