Variables have a ``slider'' feature, which allows the value of the variable (or parameter or constant) to be altered by the user via the mouse or by arrowkeys.
The options Max, Min and Step Size in the Edit form let you control the maximum and minimum values for the variable/parameter, and how much the value changes for each press of an arrowkey. A relative slider means that the step size is expressed as a fraction of max-min.
Adjusting the slider of an integral (or stock) variable while the system is running actually adjusts the present value of the variable. The sliders can also be adjusted using the keyboard arrow keys: uparrow or rightarrow increases the value of the parameter by the step size; downarrow or leftarrow reduces it.