This is a description from Wheelock's Grammar: Sixth Edition, revised (2005):
Genitive and Ablative of Description:
A noun in either the ablative or genitive case plus a modifying adjective may be employed to modify another noun; both the ablative of description and the genitive of description (...) might describe a noun by indicating its character, quality, or size, although the ablative usage was especially common in describing physical traits.
I suggest that you look in your textbook or grammar for 'ablative of description.'
As well as by Wheelock, the Descriptive Ablative is also treated (with examples) by Gildersleeve and Lodge, Latin Grammar, § 400, and by Allen and Greenough, A New Latin Grammar for Schools and Colleges (available free online with Perseus) § 415.
Edited afterthought (@CVDM): if you are a student or a beginner, the Perseus website, emanating from the Classics Faculty at Tufts University (Perseus Collections/Texts), is a very useful resource to know about.