It's fairly common on modern boats to see rope passing through a surface. Such as the deck, thwart or any other part of the hull. This rigging style ensures the rope takes the shortest path, meaning fewer turning points, less friction and less weight.
 

However, just drilling a hole through a fibreglass or carbon surface and running rope through it isn't ideal. Over time the rope will slowly cut its way through the surface - weakening the rope and making a mess of your boat.

So, fitting a through deck bush into the hole will help guide the rope through the surface whilst offering protection to the material the rope or wire is passing through. 

But, how do you know which deck bush is the right one for the job?