This sign is called Exit guide and is a type of direction sign placed where the exit begins. The sign tells you that:

The exit leads to national road no. 120 towards Nannestad and Lillestrøm.

The exit is registered as exit no. 25 (some frequently used exits have their own exit numbers, making it easier to find the correct exit).

The exit is usually informed of on earlier direction signs, so that the driver can prepare for the upcoming exit. These earlier signs may look like this: