What is open source? Well, open-source is not very hard to understand. Open Source means the product which you are calling open source has its original code free to everyone to use, redistribute, and modify. There are a lot of things that are open source, such as Rocket.Chat, Firefox, Chromium, VLC Player,Libre Office, and more. All of this software has its code open to the public. For example, Rocket Chat has its source code available to anyone at https://github.com/RocketChat/Rocket.Chat. You can download it at any time and modify it all you want. Swivro™ even has some open source projects, like Concerto Browser, and FusicTube YouTube Player. If you make your software/app/website open source, it could also help promote your website. Users could use your software, rename it, modify it, and credit your website so other people will be interested in its origin and go to your source code.
SHOULD YOU MAKE YOUR APP OPEN SOURCE?:
The answer is yes, and no. If you want to try promoting your app a bit more, and have people modify it, definitely go for open source. If you want your source code to be protected and no one to be able to modify your app, do not make your app open source.