Nonsense, rubbish.
Dismisses a wrong statement ("Quatsch!") and describes silliness in general. "Quatsch machen" is to mess around. Mild enough for any company, which makes it very useful.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to German — it uses Quatsch the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook German.
Free to start. No signup for your first conversation.