A dvd re-writer, lets you burn/write your own dvd's to blank dvd media, this could be movies and/or files and lets you read them. You can also play dvd's, but sometimes this requires extra software like the vlc player: http://www.videolan.org/vlc/index.html
Anything added to the laptop will make it weigh heavier but dvd writers are very light in laptops.
A DVD re-writer let's you watch DVDs, and it can also burn movies on blank DVDs. Some DVD's can be erased and used over and over, this was known as re-writing, and it was common before flash drives came out. DVD re-writers add a little weight, but it's not too bad.