Since April 21, 2015, Google Search expanded its use of the site’s mobile-friendliness as ranking signal. Because of this, users now can easily get relevant, high quality search results optimized for their devices. To make sure that your site is ready for mobile-friendly Google search results, below are the frequently asked questions:

How to check if my site is responsive or mobile friendly?
Google has a web app where you can check your site, here is the link : https://www.google.com/webmasters/tools/mobile-friendly/

How to make the your site responsive and mobile-friendly?
See some basic guidelines here : https://www.google.com/webmasters/tools/mobile-friendly/
If you found this too technical for you, simply contact your developer.

If my site is not yet mobile-friendly or responsive, will it affect of desktop traffic?
No. But still you are missing the opportunity to catch more traffic from the mobile device users.