"连接主句和从句的" can be translated as "connecting the main clause and the subordinate clause" or more concisely as "conjunctive (between the main clause and the subordinate clause)" when referring to the function of a word or phrase.
For example:
Conjunctions are words that connect the main clause and the subordinate clause.（连词是连接主句和从句的词。）
The relative pronoun serves as a conjunctive element between the main clause and the relative clause.（关系代词在主句和定语从句之间起到连接作用 。）