Wriiten English and spoken English are sometimes different.
When we speak, we use different vocabulary, different grammar, and different pronunciation than what is in textbooks. Here’s one example. In most textbooks, you might find a sentence like this:
                                                       “The theater sold all the tickets”

However, in normal speech, we would usually say,
                                                       “They sold out”.

"Sold out" is a two-word verb- also called a "phrasal verb". 
Adapted from Christopher Moses

What is an idiom? An idiom is two or more words with a special meaning.
Idioms are difficult, because often you can’t find them in a dictionary. For example, “to have a blast” is an idiom. What does it mean? You probably know “have”, and you may know “blast” (which means “an explosion or a bomb”)… but what does the the phrase “to have a blast” mean?
Actually it means “to have a lot of fun; to enjoy a lot”
Example: I finally tried bungee jumping, and I had a blast.