是 is the basic linking verb in Chinese, like English 'be' — it connects a subject to a noun phrase that identifies or describes it (我是学生 'I am a student'). It can also be used before a verb or adjective for emphatic confirmation ('indeed, really'), though this is less common at HSK 1. In conversation, saying 是 alone means 'yes, correct'. Always follow 是 with a noun or noun phrase, not an adjective alone.