"Horrible" means very bad, especially in a way that can harm people. Examples of things that can be described as horrible are:

• make a horrible mistake
• a horrible tragedy
• a horrible crime
• die a horrible death
• a horrible experience

"Horrible" is an ungradable adjective, meaning that it doesn't sound correct to say that something is "very horrible". That's because "very" is already part of the meaning of the word "horrible. Other examples of ungradable adjectives are:

• enormous
• exhausted
• terrible
• amazing
• delicious
• gorgeous