Things that modify nouns (adjectives, or adjectival equivalents).
Literal
Nouns [object-を] modify things (adjectives, or adjective-equivalent expressions).
A grammar textbook fragment defining noun modifiers. 形容詞 'adjective' and 形容詞相当語句 'adjectival equivalent expressions' are technical linguistic terms. The sentence is a fragment — no main predicate, just a definition with parenthetical elaboration.