Jerub-baal means he fights or will fight-Baal. It may be understood in two ways: Baal fights, or he fights Baal. The villagers mean: Baal will fight him, namely Gideon. In the story, however, the other meaning is also implied or at least suggested: Gideon is the one who fights Baal, as he did by breaking down his altar.
32 Therefore on that day Gideon was called Jerubbaal, that is to say, “Let Baal contend against him,” because he broke down his altar.