Im bad at english, what is "christen" mean?? It a slang word or something?

It has a few meanings
Typically it means "to name" someone/something in a ceremony-type situation (comes from 'a christening' of a child into a christian church ceremony)
It also is said when you "use something for the first time"

Here hes saying about using the tops office for the first time - basically he's meaning having seggs in that office for the first time

I learn something new in here loll thankyou for tge explanation