What does it mean that the 2018 iPad Pros use USB C instead of Lightning connectors?
USB C is quickly becoming a universal standard for charging and data transfer on devices.
Android phones have used it for at least a year now for most of the new phones it's a fast efficient way to transfer data and power and it allows you to do all sorts of things. USB C can be used to transfer video for example. So Android phones if they're set up correctly can use that USB C connection to display video content.
Apple has stuck with a proprietary connector for years. The Lightning connector which works great but it is different than every other manufacturer out there. With the new iPad Pros set for release in 2018 they finally bought in to USB C as the connection for power and data transfer. It allows iPad's to use a charger that you might have lying around for your phone or for your computer.
I have a MacBook Pro that uses USB C for its charging so I can use that same connector to charge my iPad Pro. It also allows the iPad Pro to send its video content out to a monitor which you haven't been able to do easily without having a number of different dongles for transferring that data. In any case USB C is a good thing for iPads because it makes them more standard like other devices and it means that you don't have to search for your charging connector if you leave yours at home. Likely someone else will have a USB C charger wherever you go.