An app - web app, or iOS/Android app, is something you give to your customers to actually use your product. Whether it is a simple smart plug, or a complex factory monitoring product with plenty of sensors and dashboards - an app always ships together with your physical devices.
Your customers register, login, and interact with their devices and data using an app. mDash provides a Customer REST API that app can use to get a restricted access for devices.
Traditionally, there two kinds of apps:
mDash can serve a PWA web app that is under your full control. By default, a web application from the Smart Light reference project (described in a Quick Start) is served, located at https://github.com/cesanta/mdash-smart-light/tree/master/mobile-app. That default web app provides customer registration, login screen, and Smart Light device control.
If you decide to implement iOS or Android native app for your customers, you could to that. Use PWA Web App as an API usage reference.