Hello ! 
From what I understood, the judges where people that God appointed during the period after the Israelites entered the Promised Land of Canaan, and before the first King was nominated. They had the duty to teach and guide God's people on the right path. They were needed because God's people tended to betray him and worship idols, which led to them becoming destroyed by the surrounding nations. 
You can read more about them in the book of Judges, for example in Judges 2:10-23. 

Thank you so much, this is really helpful ^^