Okay, so you know how sometimes there are two movies that are going to come out around the same time, like on the same weekend? And they might be kind of similar, like both action movies or both love stories? Well, counterprogramming is when the people who decide what movies to show in theaters try to pick movies that are different from each other so that people who don't like one type of movie might want to see the other one instead.
For example, let's say there's a movie coming out about a superhero who saves the world. That might be really popular with boys and people who like action movies. But maybe there's also a movie coming out on the same weekend about a girl who loves horses and has to save a farm from going out of business. That movie might be more popular with girls and people who like animal stories. By showing both of these movies in theaters at the same time, the people who decide what movies to show are trying to make sure that everyone has something they might like to see, and that they don't have to choose between two movies that are exactly the same.
So that's what counterprogramming is: it's when people who decide what movies to show in theaters try to pick different types of movies so that everyone has something they might want to see.