The mobile web apps are built using the same development tools than the web pages, so with some knowledge in HTML, Javascript, etc. you can begin to learn this kind of development. To use a web mobile app, the user must enter to any web browser installed on the mobile device and in this way the user can navigate at the URL site or URL app. One of the most important thing to know in the time of coding a web mobile app is the screen because the screen on mobile devices have multiple sizes and the user can interact with the content there in many ways, for example scrolling, zooming or doing the click action just touching the screen with one or more fingers.
You surelly must to think that one finger is more bigger than a mouse cursor, then you cannot provide little buttons in a web mobile app for example, in this way you must provide all the good ideas to make your app easy to use, for any person in any device, so the content must be fine presented on any screen, it's not matter if a user have a tablet or a smartphone, as a good developer you should consider that you information can be viewed on any screen in the best way, and how can you do it to achieve this? the answer is responsive web design RWD.
Responsive Web Design (RWD): Is an approach to web design where the web developer or web designer make a site where the informacion and interactivity is always optimal and the user can feel a good experience using the web site or mobile web app. The sites that have RWD can be viwed on diferent devices and their information in presented to the user in the best way, with a good font size, minimizing horinzontal scroll, the images are resized to fit at the width maximium screen.
Now that you have one idea on the importance of RWD, may be you could be worried about how to get started, and take hands on this important topic, I recomend that you start learning and using Bootstrap - The world most popular mobile-first and responsive front end framework.