A bit of explanation first. This is the "Ultimate Type Tool", a script that lets you do various Pokémon type-related stuff. First, you have the basics: see what happens if type X attacks some type combination, see the weaknesses and resistances of a type combination, and the offensive advantages of a type. Slightly less common is the ability to see all the type combinations that are weak or resistant to some attacking type. Then we get to outlandish when it comes to "type polygons": a generalization of the principle of "type triangles" that we know from the Fire/Water/Grass starters. Basically, it is generalized to any number of single types that can be ordered along a circle in such a way that for any two adjacent types X and Y in a clockwise direction, X attacking Y will yield the same result, and Y attacking X will yield the same result. For type triangles as we know them, specify one of the relations as "super effective" and the other as "not very effective"; it does not matter which is which. A word of warning, though: the script gets slow when the polygons have a lot of "sides", and there is a maximum number of times that the script is allowed to run through the loop that calculates them: the results are likely to be incomplete when you get to larger numbers of types.
The most interesting feature, however, is the ability to create your own type and then use it in exactly the same way as you would use a real type. Thus, you could enter the advantages and disadvantages of one or more made-up types below, and then calculate which type combinations would be weak to those types, or whether new type triangles would be posible with these new types.
Whatever you do, the "Script output" box in the top right will be where your results are shown, except that nothing is shown when you create/edit/delete a custom type. Don't mind that.
Combinations of the same type, e.g. Normal/Normal, function as a singular typing.