A clause that modifies a noun (a relative clause). In Japanese, such clauses typically precede the noun they modify.
『私が昨日買った本』の『私が昨日買った』は関係節で、後ろの名詞を修飾している。
In 'the book I bought yesterday', 'I bought yesterday' is a relative clause modifying the following noun.
英語と違って、日本語の関係節は修飾節が名詞の前に来るのが普通だ。
Unlike English, Japanese relative clauses typically come before the noun they modify.