I am new to whistles and I would like to know what the term “chiff” means.
Whistling Pops 
If you go to the main page of Chiff & Fipple that definition and others are there. Also if you are going to make a whistle you will want to read how to tweak them.
Chiff is a term used to described an air sound at the beginning of a note (the ‘attack’). However, many people use it to describe a general airiness to a tone. Your best bet is that ANY time a whistler uses this term that you get them to define what they mean when they use it.
While you’re at it, ask them how they pronounce “feadog.”