It literally translates to "by the night", but it's used in the same way as "at night" in English.
Here are some examples:
* Me gusta salir por la noche. (I like to go out at night.)
* El cielo está lleno de estrellas por la noche. (The sky is full of stars at night.)
* Voy a leer un libro por la noche. (I'm going to read a book in the evening.)