Can anyone explain what reductio ad Hitlerum means in simple English with examples?
I tried reading the Wikipedia article but it didn't help.
Reduction ad Hitlerum is just an association fallacy. As the Wikipedia article puts to an example:
Hitler was a vegetarian, so vegetarianism is wrong, because the things Hitler did were wrong, or because it could lead to results ideologically or morally aligned with Hitler.
Another example would be:
Painting pictures is wrong, because Hitler painted pictures and was a bad person, therefore by painting pictures, you will become bad person too.
It's not true, of course, therefore it's a fallacy, an incorrect argument.
Reductio ad Hitlerum references the logical method called reductio ad absurdum: carrying an idea out to a point at which it is entirely unrealistic.
Adding Hitler into the equation (reductio ad Hitlerum) just confines the absurd and fallacious conclusion to the realm of Hitleriana:
Hitler liked dogs, therefore people who like dogs are fascists.
Hitler wore a postage-stamp mustache, so I am wary of people with small mustaches.
There is an excellent TV Trope* about this called Hitler Ate Sugar.
Hitler has gained the reputation for being the very embodiment of darkest evil, who oozed "pure liquid malevolence" right out of his pores. So, he would logically only do/like/own things that are as evil as him. Things like wearing clothes, eating, taking a walk, and breathing. Hitler did those things, but that doesn't make them bad. Hitler is not a reason things are bad.
* Warning: don't go to the TV Tropes page unless you have a lot of time. You will be lucky to tear yourself away in under an hour, what with all the links to other tropes you Just Have To Read About.
Essentially, it's arguing that something you oppose is connected in some way with Hitler and that since Hitler was a bad guy, the thing you oppose must be wrong. "I'm not a vegetarian. Hitler was a vegetarian!"