if a particular stimulus in the dog's surroundings was present when the dog was given food then that stimulus could become associated with food and cause salivation on its own - from Google. If in simple terms, it is like X person doing sex with Y person under certain conditions, so when X and Y breakup, X person will be conditioned in such a way, that if they do sex with Z person, they will remember those certain conditions when X and Y did sex and might get hard. But this only happens when repeatedly done in the same exact way.
I'll try to explain
Pavlov's theory is a conditioning in where stimulus or a signal is followed by a reward.
For example:
I am exposed to food packaged in a plastic that crinkles when I open it (chips, biscuits, etc.)
So now when I hear a plastic crinkle (stimulus / signal) my mind instantly registers it as food (reward) because I'm used to seeing food whenever I hear that sound. My first thought whenever I hear it is "that's food".
In conclusion, my mind automatically associates food with that sound because I am exposed to that condition.
its like pavlovs theory but with sex