Native speakers often use slang and idioms without realizing it and without understanding they may not understood by everyone. Slang was originally the vocabulary and expressions of criminals and tramps who didn't want outsiders to understand them. If you understand each word but do not understand the meaning of a message, it may be because it is an idiom.
Idioms are expressions that have a different meaning than the exact meaning of the individual words.
Each month we'll add a new group of expressions.