This sentence comes from the fact that during the war, Lenin's guard Vasily gave his wife a loaf of bread and firmly told her that "there will be bread and milk", which means that as long as we work hard in the future! There will be a good life in the future! This sentence is usually used to motivate yourself or others!

There will be a good life. Milk and bread are daily necessities for foreign families. It means that you don't have those material things now, but through a period of development, you will have material conditions through your own efforts for a period of time. It expresses a mentality of not giving up hope.