「です」is the polite copula — it links a noun or adjective to make a polite sentence. Noun: watashi wa gakusei desu. (I am a student.) Na-adj: kirei desu. (It is beautiful.) I-adj: tanoshii desu. (It is fun.) Past: でした → Kinō wa ii tenki deshita. (It was good weather yesterday.) Negative: ではありません → Gakusei dewa arimasen. 💡 です makes sentences polite. In casual speech it may be dropped.