Here's why:
* It's impossible for words to literally freeze in the air. This is an exaggeration to emphasize just how cold it was.
* The image of thawing out words is a creative and imaginative way to show the difficulty of understanding speech in such extreme conditions.
While hyperbole is the most prominent example, there's also a touch of personification in the sentence. It gives human-like qualities to words by suggesting they have a physical form that can be frozen and thawed.