A “mark” (aka marker) means a “click”, or a word such as “YES” or even like a silent acknowledgement of a behavior for a hearing-impaired dog, like “jazz hands”. This mark is intended to work like a camera – taking a (mental) picture of exactly the behavior you like/want the dog to continue performing (I hope this makes sense?). When done correctly, the “click” will eventually elicit a Pavlovian response in your dog, which is a good thing, since that’s when the click and the treat mean the same good thing in your dog’s brain.

I hope that helps clarify a bit. Good luck with your puppy training!