The particle "が" in Japanese is primarily used to indicate the subject of a sentence. It often marks the noun that performs the action of the verb. For example, in the sentence "猫が走る" (The cat runs), "が" highlights that the cat is the one doing the running. Additionally, "が" can also be used to contrast two subjects or to introduce new information. In a sentence like "彼は学生だが、彼女は先生だ" (He is a student, but she is a teacher), "が" serves to contrast the two subjects. This dual function makes "が" an essential particle in Japanese grammar.