While using function plotter mode, this defines the the lower and upper values of X, within which the function will be plotted.
The first value defines the lower limit and the second value defined the upper limit of X.

Values can also be entered in scientific notations, like 5*10^{-12} as 5E-12

Example: If you set the lower limit to -5 and upper limit to 6, then the graph will be plotted, starting from x=-5 and plot the function upto x=6.