Prevent is to avoid. The Effort to Prevent is the effort to prevent unbeneficial qualities, those that harm ourselves and others, from arising. This is using our wisdom of cause and effect to see which situations cause problems and difficulties for us, and in order to prevent those difficulties, we try to avoid such situations or such conditions that cause unbeneficial qualities to arise. It's better to prevent than to have to try to let go all the time. So we need a certain amount of wisdom to know how to protect ourselves in the world. Otherwise if we continually allow ourselves to go into situations and conditions that are not helpful to us, we can cause ourselves a lot of suffering. The Effort to Prevent is a very important effort.