The Hugo Award is a prize given to science fiction and fantasy works that are really good. A "semiprozine" is a magazine about science fiction and fantasy that is made by people who aren't only doing it for money. They might still make some money from selling the magazine, but they also really love what they're doing.
So when someone wins the Hugo Award for Best Semiprozine, it means that their magazine about science fiction and fantasy was the best one made by people who aren't just doing it for money. It's like getting a gold star for making a really awesome science fiction and fantasy magazine with your friends.