Plain copula 'to be' used in informal/plain speech to state that something or someone is in a particular state (equivalent to 'is/are').
これは本だ。
This is a book.
彼は先生だと思っていたが、実は学生だった。
I thought he was a teacher, but he was actually a student.
明日は休みだ。映画を見に行こう。
Tomorrow is a holiday. Let's go see a movie.
そんなことを言うなんて、君は変だ。
To say something like that, you're strange.