Thanks to ‘The Wine Guy’, Bob Trimble for an online thread he posted on this topic. I had to actually look it up as it wasn’t familiar to me. Though I’m usually the last one to admit it, I don’t know everything.
Pet-Nat (or Pet’Nat) is short for Petillant Naturel. It means natural sparkling wine that is produced with little or no additives. Petillant Naturel wines are bottled during the primary fermentation as opposed to more traditional sparklers like Champagne or Prosecco, which go through a second bottling to produce the bubbles. This process captures the natural CO2 from the wine. Only the native sugars and yeast from the grapes are present in production of Pet-Nat.