A significant portion of the text on the page is too small relative to the width of the page. This makes the text hard to read on a mobile device. Look at the test screenshot for your device to try to identify the problematic text.
To fix: Specify a viewport for your web pages and set all your font sizes to scale properly within the viewport, so the text will be visible on a device screen. Read more about best practices for font size.
Font sizes less than 12px are too small to be legible and require mobile visitors to “pinch to zoom” in order to read. Strive to have >60% of page text ≥12px.
As reported by Google
As reported by PageSpeed Insights