iTunes keeps a catalog of each item (movie, TV episode, song) so moving things to another drive involves 2 steps: 1) moving them to another location and 2) updating iTunes.
If you want to move everything in your iTunes catalog to the external drive the solution is simple and this article explains how. If you just want to move the movies then you'll need to do the following:
1) Copy your movies to your external drive. It would probably be best to create a folder on the drive called Movies. (An easy way to find the folder where iTunes keeps the movies is to open iTunes, select a movie and then from the File menu select Show in Finder.
2) In iTunes, delete all the references to the movies you've just moved.
3) Drag to folder containing the movies onto the iTunes window. This will recatalog the movies so iTunes knows where they are