For many of our clients we use Google Webmaster Tools and Google Analytics to see who is coming to the site, where they are coming from, what they are doing when they get there, and other user related data. Recently, we have received some emails from Google regarding a few client sites that are older and not "responsive". Meaning the website interface does not adjust to the device the user is viewing the site on, making it more difficult to use and hurting the user experience on a phone or tablet. The emails we received from Google were sites that are older and were created before responsive sites were best practices and mobile viewing of websites was limited.
What this likely means is Google will potentially penalize you in search rankings if your site is not "mobile friendly". They have been hinting at that for awhile.
Know the warning signs
Masonry really likes that Google is sending out these warnings. Google's objective with the organic rankings is to present the "best" sites first because that gives searchers the best experience when searching a topic and if searchers have a good experience, they'll be back. Quality websites are consistently being worked on, added to, and generally maintained. They also are built professionally with best practices.
So in this day of mobile devices, it is a best practice for a site to present well on these mobile devices.
The issue though, as mentioned above, is that an older site may have been built well and maintained properly but not be mobile friendly simply because that technology did not exist when the site was originally created. The good news is that there are solutions for this. The bad news is that it is likely not a quick fix.
The best option but not always a realistic one is a complete rewrite of the site with a mobile first design built on a responsive framework. The issue here is that it is likely going to be the most expensive solution and a customer may not have the budget for what is essentially a new site.
Another option is to just do your best by adjusting the code as it is and adding in media queries with breakpoints for a tablet view and phone view. The amount of time and effort to go this route can vary greatly depending on how the original code was written.
Regardless, Google is strongly recommending that you make your site "mobile friendly" one way or another.
Again, in our opinion, even though this creates some work and is not necessarily easy, it is well worth it. Mobile devices are not going anywhere and improving your user's experience on your website while improving your search ranking at the same time are good for both you and your customers.