This about is taken directly from the JQuery Mobile website:

http://jquerymobile.com/about/

"jQuery Mobile is a HTML5-based user interface system designed to make responsive web sites and apps that are accessible on all smartphone, tablet and desktop devices. It is built on the rock-solid jQuery and jQuery UI foundation, and offers Ajax navigation with page transitions, touch events, and various widgets. Its lightweight code is built with progressive enhancement, and has a flexible, easily themeable design." - JQuery Mobile team

I would strongly recommend going to the site to learn more about JQuery Mobile. This is just an excerpt to help beginners.

You can also find more examples and information on the jquery-mobile official page: https://jquerymobile.com/demos