What is it called when it's said, you HAVE to have bad to know good? is that some bullshit or what?

In short, yes, that is "bullshit." While I can't pin this idea to a specific fallacy, I can say that the general form of this is an unsubstantiated claim . The best response to such a claim, is "okay, back up that claim with evidence, reason, or logic." The burden is on them (since they made the claim), so don't let them tell you to explain how one can know good without knowing bad.

Well, can you prove what good is without using bad? Can you define good without having something to compare it to as an opposite to understand what is meant by good? Can something be true if there is no such thing as something being false?
Can you have darkness without light existing? By that I mean, define darkness without using the word light. Did someone invent the word light without it being applied to an observed thing? If darkness needs the word light in order to define it, as every dictionary I've seen does, then doesn't that mean that we have to know what light is for there to be an absence of it?, The definition of darkness Webster's says...
\ˈdärk\
adjective
: having very little or no light
: not light in color : of a color that is closer to black than white
of a color : having more black than white : not light
Full Definition
1 a :devoid or partially devoid of light :not receiving, reflecting, transmitting, or radiating light <a dark room>
b :transmitting only a portion of light <dark glasses>
2 a :wholly or partially black <dark clothing>
b of a color :of low or very low lightness
c :being less light in color than other substances of the same kind <

Sure are a lot of references to the word light. So ask yourself is good and bad analogous to light and dark, true and false. If not then you can define good in a way that doesn't need bad as something to contrast it to, therefore you do not have to know bad to know good.