Relating to the north or northern regions.
Septentrional is a fancy way to say 'northern'.
Examples:
The septentrional part of the country is very cold.
I want to visit the septentrional regions of the world.
The septentrional winds bring cold air.
The septentrional lights are very beautiful.
In the septentrional lands, it's always snowy.
The septentrional animals have thick fur to keep warm.