What does it mean if you have pyrosis?

It means you have heartburn.

Pyrosis is a burning sensation in the chest that sometimes radiates to the back of the throat. The cause is stomach acid refluxing (flowing back up) into the esophagus. Normally, a ring of muscle called the lower esophageal sphincter (LES) keeps stomach acids in the stomach where they belong. But if this sphincter relaxes too much or opens too frequently, stomach acids can flow back up into the esophagus, causing pyrosis.