What is this express transportation thing ?!

I’ve read about people using/not using it. What is express transportation. How much? How do you get it? Where do you get it? Thanks for any information

In short, it’s an add-on service ($19 for one day hoppers, or $29 for a 7 days with multi-day hopper and APs) they gives you park-to-park transportation from within the park gates… this means that once you do the bag check and tapstiles once, you can avoid them all day as long as you never actually leave the parks.

I am seriously considering it for my next trip. It would be invaluable if trying to do a 4 park day…