Ropes are tools you receive later in the game. They act like springs, but are much more flexible. They will also break with to much force applied by one side, or when hitting a rock or an object. Strings can also be broken by pressing the cut rope sign.
Use the ropes when you have a two part vehicle. You can also cut the ropes.
You can also use both of the airplane parts attached onto a box with the piggie in that box with a rope connected to a car.
You can build an airplane but add a rope to the bottom with a box/car with the piggie in it and use that to get to some elevated platforms in Field of Dreams.