Translation:It is annoying when you are yelling.
It means the same as the expression "lo riega" in Spanish. "It sucks" would mean the same in English, and though most people don't think about the origin of that expression, and is considered completely inoffensive by a whole lot of people, as noted below, some people might consider it crude so some of the other options below. "It's a bummer is a weaker expression, but carries a lot of the same meaning, and would not offend even your grandmother, or anybody else for that matter.
"It sucks" is probably the best translation of "Det er nederen", though.
nederen is very informal and is mainly used by the younger generations. The dictionary translates it as "damned frustrating, very annoying".
To be honest, it's the kind of word you need to hear a lot in order to internalise how it's used.