Gibberish is a meaningless collection of words. Here is a short example:
I know I withought all by he middle on," said a between he name a buzzing, he for began open he the only reason for making. You don't quite sure," sang a growly reason for its making to he being-noise middle on," said to he name of then place upon at I am," said: "That I am," sat does he the does?
It is not slang.
If I was reading something and found that it was really difficult to understand because the English used was poor and the logic was confused, I might throw it onto the floor and exclaim, "That's gibberish!"
Oh - I didn't notice this is your first post. Welcome to these forums elemay20.