Google announced that all sites that do not support mobile-friendly pages or apps will suffer from decreasing search ranking since 21 of April. Read the full information in Google's official blog.
Q: Why should I care?
A: Sites' positions in search results are determined based on hundreds of factors designed to provide end-users with helpful, accurate search results. The lower you stay in ranking, less people could see your site. For more information see How search works
Q: Is my site mobile-friendly?
A: You can check immediately in Google Webmaster tools here.
Q: Is this blog mobile-friendly?
A: Yes! See the result on your own:
Q: Need help! My Sitefinity site did not pass the mobile-friendly test.
A: Luckily Sitefinity provides responsive web design via the Responsive design module.
Feel free to contact me on firstname.lastname@example.org for a consulting on best SEO practises for your Sitefinity site or how to create responsive web design.