Perhaps the OP is using one of these apps:
BSPlayer has a pop up window option. If enabled, one way to get to it is to hit the back button while the video is playing, not paused (easy to do and can cause confusion the first few times you do it). The window is movable and plays on top of other apps.
Popcorn Player (free) is intended specifically to playback in a pop up, though it will go full screen. It's resizeable with a movable window that will show on top of other apps.
Stick It! (mentioned by TomsAndroid) is similar to Popcorn Player, but is a paid app.
Sent from my Nexus 7 using Android Central Forums