The monorail has two stops: Tomorrowland (inside DL) and DTD. You must have DL park admission to ride the monorail if you get on at DTD. Also, it is a one-way ride if you get on at DTD. If you board in Tomorrowland, you can ride round-trip, but must get off once you return to Tomorrowland. It does go through part of DCA and the Grand Californian (but unlike the Contemporary at WDW, it stays on the outside the hotel).
They will check your bags and scan your ticket at the DTD station, just as if you were entering the park through the main entrance.
It's a fun way to go from DL to DTD if you are leaving the park to go to a Disney hotel or if you are planning to eat, shop, or see a movie at DTD.