It means you can have Apple Maps / Google Maps / Waze etc as your sat NAV in the car. Makes finding destinations much easier (ask Siri) and the map on the screen is much better than the Lexus one. It also integrates Music and Messages app.
Because of the slight faff of having to plug into USB port, some people get the converter discussed above that gives a permanent usb connection nad syncs wirelessly to your phone.
I’ll probably get one of those at some point, but for now tend not to bother with car play for short day to day activities, but load it up going somewhere unfamiliar, further away, or where traffic might be a concern.