Generally, a value judgment is a bad thing. It means saying that a person's actions are inherently good or (more commonly) inherently bad--especially when this is not true (or at least, not certain, or not necessarily so). It is closely related to a stereotype, although stereotypes are usually caused by ignorance or lack of thought; value judgments tend to have at least some thought put into them.
The only people really qualified to make value judgments are judges and lawmakers (gods also fall into this category), but their jurisdiction in these matters is not universally recognized.