An idiom is when something is said, but is not supposed to be taken literally. Like, "I held my breath forever..." doesn't mean that they actually held their breath forever and isn't supposed to be taken literally. Some other examples are: raining cats and dogs, he got cold feet, etc.
In the sentence, we see the word "forever". Realistically, no one can hold their breath for forever. But that was an <em>exaggeration</em>. Hyperbole just means an exaggeration.
