Like you are on a webpage, let's say MR forums, and you pinch to zoom on letters. I could see some jaggedness on the outline of the text when zoomed in on the iPhone 6 but on the 6+ the text still appears smooth and sharp.
That makes no sense as you are not zooming in on the screen itself. If anything you'd need to zoom out to make the letters smaller to see if they maintain clarity or become little blobs.
I'm surprised this pinch to zoom actually convinces people they are testing screen clarity. I'm even more surprised Apple once used it in advertising to show just that. How is spreading a single letter across many pixels any indication of quality? It isn't.