How can I watch movies offline on my Android tablet?
Android comes with a basic video player called "Movies", although it might not show in the application list for some devices. To use the built-in player, you need to use the Gallery or a file manager to invoke the Movies app on the movie file you want to play.

Alternatively there are many alternative movie players on the Market. Here are a few threads to get you started: 


MX Video Player
MoboPlayer
RockPlayer
QQPlayer
VPlayer