This might help to explain it. (copied from wikipedia)
"A mobile application or app is a computer program or software application designed to run on a mobile device such as a phone, tablet, or watch. Mobile applications often stand in contrast to desktop applications which are designed to run on desktop computers, and web applications which run in mobile web browsers rather than directly on the mobile device.
Apps were originally intended for productivity assistance such as email, calendar, and contact databases, but the public demand for apps caused rapid expansion into other areas such as mobile games, factory automation, GPS and location-based services, order-tracking, and ticket purchases, so that there are now millions of apps available. Many apps require Internet access. Apps are generally downloaded from app stores, which are a type of digital distribution platforms. "
I usually play on RHP using a laptop, but I do have an iphone and have installed the RHP app on that, which allows me to access the site and check to see if I have moves waiting, in cases when I'm away from my laptop.
Using the app has limited features, as compared to using a regular browser such as firefox to access the site.
(I think there may be a menu option on the app that allows you to use more of the site options, but I have not tried that so far. Perhaps those who use the app can provide further details. I manage to make moves in my games using the app, and that's the extent of my usage from my phone.)