It's a contraction of "no se sabe" (one doesn't know) and is a common phrase used in everyday conversation.
Here are some examples:
* "¿Sabes dónde está la biblioteca?" - "No se." ("Do you know where the library is?" - "I don't know.")
* "No se por qué hace esto." ("I don't know why he does this.")
* "No se lo que va a pasar." ("I don't know what's going to happen.")
It's important to note that the meaning of "no se" can vary slightly depending on the context. For instance, it can also be used to express uncertainty or ignorance about something.