A simile is a kind of figurative language used to compare two objects or ideas using the terms "like" or "as". It is used to exaggerate the meaning or to bring a certain point across about the object. In your example, "when I began reading a book it was like a bowl of letter soup, " it compares when you first started reading to a bowl of letter soup which emphasizes how first learning to read is a bit confusing since all you seem to see is a bunch of jumbled letters.
