A question about #indef statement in Adafruit backpack sketch

Hi all,
I was playing with the Adafruit 4-dig 7-Seg display library today and I noticed this code (see below) within the SETUP function.

At first glance - I interpreted it to be an IF-THEN statement - to the affect that if the Arduino device is an ATtiny85 - then setup USART serial and print to the Serial Monitor.

However when running it - with a Nano - it sets up the USART serial and prints to the serial monitor anyway.
So I obviously don't understand this statement.

Can anyone tell me what it does?
Thanks!

#ifndef AVR_ATtiny85
Serial.begin(9600);
Serial.println("7 Segment Backpack Test");
#endif

#ifndef = if NOT defined

AH! Thank you! :-]

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