For this information you have to ask the SIM Provider:
serial_number    =   serial number of the station
country_name    =   country of the provider
MCC                      =   MCC of the provider
MNC                     =   MNC of the provider
apn                       =   APN server
user_name         =   user name of APN server
password            =   password of APN server

How to set the APN in PI Service Terminal:

Please connect the station to PI service Terminal and go to the MAIN MENU (press H):
Then select "P - Setup NB-IoT module parameters"
Then please select "3 - APN NAME" 
And then insert the new APN:

The APN has been introduced by some providers shortly, until recently it was able without APN.