This imaginary dartboard is a way to visually compare two probabilities or quantities. Like a pie chart, the areas are in proportion to the values supplied. But in this metaphor, the areas are split up and distributed more evenly across the circle. (Assume random unskilled dart throws.)
Enter any two positive values. (They do not need to total 1.0 or 100.) Example: if there are four balls in a bag, one red and three blue, so that choosing a ball has outcomes with a 1/4 and 3/4 chance, you could enter 1 and 3, or 25 and 75, or .25 and .75.