「+」。
Relative pronouns serve the function of a conjunction plus a pronoun.
Literal
Relative-pronouns [topic-は] "conjunction + pronoun" [genitive-の] function [object-を] are-doing.
Another English grammar explanation in Japanese. 関係代名詞 (かんけいだいめいし, 'relative pronouns'), 接続詞 (せつぞくし, 'conjunction'), and 代名詞 (だいめいし, 'pronoun') are standard grammar terms. はたらき (from 働く, 'to work/function') means 'function / role' in a grammatical context. ~をしている ('is doing / serves as') describes the ongoing function. A metalinguistic sentence used in English-language instruction.