This term goes hand in hand with the previous type of apps: A multiplatform app is a set of programming rules that are compiled together to be used both in the Android environment and in the iOS environment.
There are several tools and frameworks that allow us to perform this operation. The most common, the environment of Xamarin and React Native. It should be noted that react native is the base of the Facebook app.
The idea of using Xamarin is to use .NET as a platform to exchange information so that when compiled onto a common language, it can be implemented in both Android and the iOS platform.
The main advantage of using a cross-platform app is that you export to both mobile platforms without problems, making only some compatibility adjustments. The main disadvantage is that you need to know a good standard in the use of programming languages to avoid incompatibilities when exporting.