The particle "는" is a subject marker in the Korean language. It is used to indicate the subject of a sentence, especially when the subject is being contrasted or emphasized. For example, in the sentence "나는" (I), "는" highlights that "I" is the main focus of the statement. When used with nouns, "는" attaches to the noun to clarify that it is the subject. For instance, in "사과는" (the apple), "는" shows that the apple is the subject being discussed. This particle helps to structure sentences and convey meaning clearly in Korean communication.