To signal to browsers that your page will adapt to all devices, add a meta tag to the head of the document:
<meta name="viewport" content="width=device-width, initial-scale=1.0">
The meta viewport tag gives the browser instructions on how to adjust the dimensions and scaling of the page to the width of the device. When the meta viewport element is absent, mobile browsers default to rendering the page at a desktop screen width (usually about 980px, though this varies across devices). Mobile browsers then try to make the content look better by increasing font sizes and either scaling the content to fit the screen or showing only the part of the content that fits within the screen.
For users, this means that font sizes may have an inconsistent appearance, and users may have to double-tap or pinch-to-zoom in order to be able to see and interact with the content. For Google, we might not judge a page as mobile-friendly because it requires this kind (or type) of interaction on a mobile device.