In the years past, just having a web presence was all you needed. That theory is now old school and no longer valid. Now, sites need to have an SSL certificate and be totally responsive.
What does it mean to have a responsive website?
It means that your site is going to respond to the size of the screen that it is being viewed on. Example: if you bring up your site on your computer, it probably fills the screen good enough. Bring it up on your phone or tablet…do you have to pinch and zoom to read the content? Bring this site up on your phone or table and you will see that the site actually responds to the size of your device and you do not have to pinch and zoom…it looks full screen and is totally readable. In English, it means that your site will work on all mobile devices without having to do the old fashioned pinch and zoom.
Can you take a site and convert it to be responsive?
Yes, but the amount of labor involved would cost you more than just completely redoing the site with new code.