How does a 3D TV work?

The image is actually two different images slightly offset from each other. If you look at it directly, it looks blurred or like a comic book that was printed poorly.

The viewer must put on "shutter glasses". These are glasses that will allow only one eye to view in sync with the TV picture. When the left picture is on the screen, the glasses let the left eye view. When the right pic is on the screen, the glasses let the right eye view.

Now days, typically, the shutters are LCDs that will either block or pass the light from the TV. The syncing is done either optically or via Bluetooth so the shutter glasses will open and close in sync with the image on the TV