They're like variables that you can modify manually, using the mouse.
For example, a good candidate for a slider is the speed of the boat in a math simulation. Say you have a slider called SPEED. In a procedure, you would use:
forever [forward speed wait 1]
instead of:
forever [forward 1 wait 1]
Then you can use the slider to control the speed of the boat.
Another example is a number of times to throw a dice. Say you have a slider called TIMES, and a procedure ThrowTwo that throws two dice, once. In a procedure, you would use:
to experiment
repeat times [ThrowTwo]
end
to ThrowOne
talkto [t1 t2]
setshape 1 + random 6
end