In the beginning of a ride, the app will let you chose between several routes. However, even if you purposely selected a certain route, it will automatically reroute you, if it found a "faster" route. Sometimes though, you do not want that because you chose a certain route on purpose. Unfortunately, other than in Google Maps (where you get a notification that a faster route was found that you can dismiss) there is no option in Waze to reject such rerouting. This can sometimes even lead to dangerous situations, like if you get rerouted through an unsafe neighborhood.