This is the 87th most frequent German word.
sondern
"Instead" or "but rather" (used to contrast).
Here, 'sondern' is used to contrast two mutually exclusive states or ideas, emphasizing that one is true instead of the other.
Er ist nicht müde, sondern konzentriert.
He is not tired but rather focused.
The word 'sondern' distinguishes the two items, specifying what is actually being referred to as a preference or truth over the other.
Ich esse keinen Fisch, sondern Gemüse.
I don't eat fish, but rather vegetables.
In this case, 'sondern' points out an alternative destination, clarifying the correct information.