I want to know what people mean when they say "Ah yeah, don't listen to him, he is a Democrat" (as if it was an insult!), why? Why should I listen to you, I want to decide. Also, please try to recommend books with the less bias from any view possible.

My knowledge on the topic is internet knowledge, if I hear communism I think of Marx and consider it bad, even though I know nothing about him, and if I hear capitalism, I think about insatiable pigs and consider them bad also, I wanna change all this and have a critical point of view! ( these are the things that come to my mind based on heuristics of thinking, it does not mean that I consider them this way, but those are the things which come to my mind)

Thx you for your time. Lectures, videos and anything else is accepted!