[jordanlyall]

What the writer is referring to is the "Click Through Rate". A "conversion" is when the user takes an action post click (submits info, downloads, purchases, etc.).

He also incorrectly calculate the CTR. The CTR on an email campaign is clicks divided by opens. He divided clicks over sends, which since fewer people opened the "Happy Holidays" email, fewer would have naturally clicked. "Merry Christmas" still performed better, but not at the rates he's suggesting.