Do you have a typical platform dependent issue you're battling with ? Ask it here. Make sure you mention your platform, compiler, and wxWidgets version.
I think backend is a wxWidgets platform-specific implementation (class), which in turn uses the platform specific API.
The backend is used also for example in wxMediaCtrl and wxWebView. However, these two are a bit different since they can use several backends on the same platform, at least on Windows.
In the past I used WebView and the backend issue arose there, and this is why implying a backend by API was not clear. I guess different developers use different terms. Maybe the doc needs improvement.