Green screen is when a giant green screen is used to project images behind whatever's standing in front of it. It's a technology that has been used in gorgeous, multi-million dollar movie and video productions, and when done well it looks great. When done poorly though, you get things like these 10 videos.
Damage - "Forever"
It's literally 4+ minutes of some weird swirling blue background. Why?
Backstreet Boys - "Larger Than Life"
This video looked amazing back in the 90s, but the CG from those years has aged REALLY badly.
Twenty 4 Seven - "Slave To The Music"
Solid color backgrounds with random words flying by? Why did they even need to use green screen for this?
Swoop - "Apple Eyes"
They're on a checkerboard... in an orchard... in the sky? What?