This is the 2579th most frequent German word.
altes
"Altes" means "old" in English, referring to something aged or not new.
Here, 'altes' functions as an attributive adjective describing the noun 'Buch' (book). It is inflected to match the gender, number, and case of the noun it modifies, as dictated by German grammar rules.
Das ist ein altes Buch.
That is an old book.
In this sentence, 'altes' again serves as an attributive adjective modifying the noun 'Foto' (photo) in the accusative case, matching its gender and case forms.
Ich habe ein altes Foto gefunden.
I found an old photo.
Here, 'alte' (a variation of 'altes') is used as an attributive adjective to describe 'Zimmer' (room) in the nominative case, showing inflectional agreement with the noun's gender and case in context.