It all started with a slice and a camera.

At least that's how Barstool Sports CEO Erika Nardini describes the birth of One Bite, her company's pizza-reviewing app that has seen a meteoric rise since Barstool Sports founder Dave Portnoy started casually reviewing pizza parlors.

"Dave likes to talk about pizza, so that's really where it started, where Dave was doing pizza reviews that were getting hundreds of thousands, if not millions, of views," Nardini told CNBC in a joint "Mad Money" interview with Portnoy on Thursday.

It didn't take long for the app to take off. Within hours of announcing the test, or beta, version, 30,000 people had been placed on a waitlist to download the app. One Bite saw 70,000 downloads on the first day of its official launch — that number grew to 125,000 by day two.

Viewers that have flocked to the app, which has partnered with Slice to let users order pizza on its platform, include "people who are never going to go to [that] pizza parlor" and "people who had never heard of Barstool Sports," Nardini told "Mad Money" host .

"He walks down the street and he's the pizza guy. And what people asked him about was, 'How can I find all the pizza places?'" she said. "We're helping people discover pizza. There's no uniform database for pizza in the U.S. Why shouldn't that be from us?"

And Portnoy, who founded the privately-held Barstool Sports as a sports-oriented newspaper and has helped grow it into a sprawling, satirical, social-media-savvy sports-and-culture platform, has even bigger dreams for One Bite.

"Yes, we want to stick with pizza. It is my favorite food," he said. "But I think the natural jump is you could go with Chinese food, Mexican, anything."

Watch out, Yelp. (Yelp declined to comment on this story.)