Re-Written or re-edited to be suited with an American audience in mind. Contrary to popular myth, this does not mean making something worse. However, if something was never good to begin with, then the editing process may do a favor only in not glorifying activities which ought to be illegal all over the world indisputably. It protects all viewers (not just American children) from senseless, stupid glorification of pure evil.
However, defending viewers from the shallowness and near-criminal perversion of the original incarnation does not promise that the edited end product will be good; only that it is less terrible, and therefore, more offensive to perverted snobs who think man-on-man porn should be viewed by toddlers - and then practiced on them!
Americanization of foreign animation often does something to original content that is a dis-service: over-simplifying the themes of the original. The need to conform things to a very simplistic GI Joe vs. Mad Scientist/Neo-Nazis format is a narrative tradition that formed after WWII, and traces of it remain in American writing today as die-hards.
In most anything else:
Altering something foreign to the US to make it more practical and digestible to American consumers. (Or depending on what gets altered, making it more universally edifying or better suited to anyone anywhere with any ounce of self-respect.)
Power Rangers is an Americanized reinterpretation of the Super Sentai. It is better, not worse, in the sense that it does not glorify lifting up skirts or grabbing breasts (sexual harassment) as the Super Sentai does. (Beware of anyone who prefers the pro-harassment content - they might live in your neighborhood and have predatory eyes on your sister!) But it dumbs down the evil characters and over-dubs poorly-written dialog in the place of the original dialog.
Americanized food packaging is easier on the eyes of anyone who is not from the Far East. But with extra preservatives in the food, the health improvements are debatable.
Anime or Manga that has been edited so as to not include what the authors originally intended. This editing is to make it possible to send this product into the minds of small children who aren't smart enough to understand americanization makes the product crappy and are unable to get it the proper way, straight from Japan.
Yu-Gi-Oh is what happens when americanization gets out of hand.