It is just like the date on a container of cough medicine. I just finished using up the last of the Nyquil that had an expiration date of 2016. It works up till 2016 so well that I didn't need it until last week when I came down with Covid. Having nothing else I used it and it still worked as well as it did in 2016. I have a new bottle to start tonight, and that one expires sometime in 2024. I hope that I don't need it any more often than the last bottle that I had. If it is fresh food then toss it at the expiration date. If it is a sealed bottle of medicine, toss it or take it at your own risk and peril.
I have no problem using some types of medicine past the expiration date. If your NyQuil looks cloudy, or discolored (from the original color), contains any floating particles, or has an unusual odor do not use the product.