If you ever tried to browse the web with a mobile device only to enter a site that has very small fonts and incredibly small and awkward menu items, this is happening because the website you are navigating is not responsive. Responsive design is basically the way to create websites that automatically molds the size of the site to the current display being used. It makes fonts larger and it aligns images and videos properly to fit smaller screens.
Your website needs a responsive design because many visitors would rather move on to the next option if they can’t navigate your site comfortably and without any issues.