Hyperbole is a figure of speech that involves exaggerated statements or claims not meant to be taken literally.
The two examples you provided that qualify as hyperbole are:
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There were two billion emails waiting in my inbox when I returned from vacation. - This is an exaggerated statement about the number of emails.
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The Ranch House on Oak Street has to be the worst restaurant in the universe. - This uses extreme exaggeration to emphasize how bad the restaurant is perceived to be.
Both statements are examples of hyperbole.