plz paraphrase it

"to stand out" means "to attract attention", "to stick out".
I'm no native speaker, but here my try of a paraphrasing: "There is no strict definition of "that guy" but normally someone is called "that guy" when he unwillingly attracts attention because of unfortunate reasons."

That person sets an eye catching example to avoid. I'd say an offbeat or ill-mannered or ridiculous person

'Stands out' is the opposite of 'blends in.' For instance, all the dark moths blend in with the dark, sooty tree bark when they are resting on a tree trunk, but the white moths stand out against the sooty bark, and are eaten by birds more often.