what does "red line" means

Can't help with a translation. To "red line" means to exclude. It is most often used in referring to the practices of bank loan officers and insurance companies who will not provide services to certain customers because of where they live. It usually excludes racial or ethnic minorities. The term comes from the oldest example of the practice, in which a red line would be drawn on a map, and bank loan officers or insurance agents would be told not to make loans or sell insurance to people with addresses inside the red line.

It's use here is rather obscure, though. Although it is used to mean an exclusionary practice, the implication is almost universally held in English to mean excluding minorities from participating economically. I don't understand what this author means. I suspect the author doesn't understand what he or she means, either.

In this sentence, I think it means a danger line, a place where there will be consequences if you cross it. Like if you drive a car to the red line on the tach, you risk the engine.

So, something forbidden, or highly inadvisable.

that helps a lot~~thanks ，guys