A 'slider' is like one of those keychain games, (they make big ones too ) a lot of little interlocking squares, with one empty spot, and you slide the squares every which way to form a picture, or message, or sometimes just line up numbers or the alphabet. Lots of people confuse a jig-saw puzzle with a slider, but with a slider you can NOT pick up the pieces or rotate or turn them over to move them. You must, what else? SLIDE them around.
The most notorious I know of is the one at the end of the 'Mayan' world in TimeLapse. It had FOUR sliders in one frame, if you got one scene, it locked in place, so you had to work around that part to continue with the rest. Your space to work in got smaller and smaller. Soooo much fun!!