This is the 17729th most frequent Russian word.
ужасное
"Ужасное" means "terrible" or "horrible" in English.
Here, "ужасное" is used as an adjective in the nominative case to describe the noun "зрелище" (sight), emphasizing its horrifying or dreadful quality.
Это ужасное зрелище.
This is a terrible sight.
In this sentence, "ужасную" is the accusative case feminine form of "ужасное", modifying "музыка" (music), and it conveys a very negative opinion about it.
Я никогда не слышал такую ужасную музыку.
I have never heard such awful music.
"Ужасное" also modifies "поведение" (behavior) in this sentence, illustrating its extremely bad or undesirable nature.