Marker
A sound — a click, or the word "yes" — that tells the dog the exact instant it got it right, and that payment is coming.
Also called: clicker · bridge · yes · marker word
Timing is the whole difficulty of training a species that cannot be told what it did wrong. By the time a treat reaches a dog's mouth, two seconds have passed and the dog has done four other things. A marker freezes the correct moment so the reward can arrive late without confusing anyone.
It only works if the sound has been paid off consistently. A marker followed by nothing a few times is just a noise, and rebuilding it takes longer than establishing it did.
A clicker and the word "yes" do the same job. The clicker is more precise and never sounds annoyed; your voice is always in your pocket.