In grammar, "secondary" or "subordinate" refers to clauses that cannot stand alone as complete sentences. These clauses provide additional information to the main clause but rely on it for context. For example, in the sentence "I went to the store because I needed milk," the phrase "because I needed milk" is a subordinate clause. In other contexts, "secondary" can describe something that is not primary or main. For instance, in education, secondary education typically refers to schooling that follows primary education, such as high school. This level of education prepares students for higher education or vocational training.