En route is another way to say "on the way." If you want to get a milkshake on your way home from work, you might suggest, "Let's stop at the ice cream shop en route!"
You can use en route when you've embarked on a trip: "My train has left, and I'm en route to Milwaukee." Since it's a French phrase, en route is a sophisticated way to express the fact that you've set off or that you're on your way. The correct pronunciation sounds like "on root."