VLC is an application on your Mac that is used to play DVDs. It's just like a DVD player. But it has a few additional tricks up it's sleeve that may come in useful in the classroom.
VLC allows you to capture and record a segment, part or section of a DVD so that you have a downloaded copy of that file on your computer. This is great for being able to share quick clips of something with your students without having to fire up the DVD player just for one small segment of video.