The message is that when you're on drugs, anyone looking at you can tell, because drugs make your eyes look funny. So if you're driving while under the influence, the police will know. By looking at your eyes. So, don't.
This is not true. It's obviously not true. Anyone who's ever seen someone on drugs will know that they don't cause your eyes to become the size of golfballs - the advert uses image morphing to make the eyes enormous, as explained in the Making Of clip:
Some drugs do have subtle effects on the eyes, such as pupil dilation, but you can only spot this if you're staring someone right in the face from about six inches away. Not to mention that anyone who's used a car will know that drivers don't spend their time examining the eyes of their fellow road users.
There's one very good reason why you shouldn't drive on drugs, which is that you might crash and kill yourself or someone else. Why the Department of Transport didn't use this as the basis of their advert, only they know. As it is, they've ended up with something that absolutely no-one is going to take seriously - see the YouTube comments. When YouTubers are making incisive criticisms of your campaign, you know you're doing something wrong.