By default, this is a modern sans-serif font hardcoded into the engine. It will be automatically created and freed if needed. This is comfortable for simple applications, you can just start "drawing text" without initializing anything.
You can assign your own font here, to make this the default font used by all 2D controls.
Note that assigning font here does not make it automatically freed (this would cause more trouble than comfort). To make sure your own fonts are always freed, set the font "owner" at creation, e.g. the example below sets the Application as owner:
It is especially useful on Android, where we can lose the OpenGL context at any moment, as user may switch applications in the middle of the game. When getting back to the application, we need to initiailize some resources, and during this process we also show this image. So this serves as a universal "please wait, we're loading" screen.
Note that the customization of this image should be done before Application.OnInitialize has started, so it has to be usually done from the "initialization" section of some unit. And in the "initialization" section of a unit, you cannot load files (doing LoadImage at this point may fail on some Android devices, as we cannot load assets before activity is started). So you can only assign images already available in code — use image-to-pascal tool to convert any image to a Pascal code for this purpose.