https://www.google.com/webmasters/tools/mobile-friendly/
Typical of the problems you might see is that your text is too small to read, the links are too close to easily touch with a finger without getting the wrong link, and your images are too wide.
I am not a coder, but I do hand code my web pages. So, I had to find some very simple solutions. Here is what worked for me to fix the above problems.
First, insert this text in the header:
<meta name="viewport" content="width=device-width, initial-scale=1.0">
This meta viewport tag gives the browser instructions on how to adjust the dimensions and scaling of the page to the width of the device.
Second, I just added breaks between links that were too close.
Third, this CSS code in the HTML header takes care of the image scaling:
<style>
img {max-width: 100%}
</style>
I don't use separate style sheets, so just inserting this in the HTML page header was a simple solution.
Again, I'm not a coder, and I'm sure many on this forum are much more sophisticated about this. But, these modifications to the webpages I write have, so far, allowed them to pass the Google "mobile friendly" test, which could be increasingly important to be found on searches.
Paul
PaulRoark.com -- Paul Roark's Photographic Home