Can someone explain how and when to use DECLARE DLLs?

I see the following code in the sample Tastrade app and don't
understand it.

*-- DECLARE DLL statements for reading/writing to private INI files
DECLARE INTEGER GetPrivateProfileString IN Win32API AS GetPrivStr ;
String cSection, String cKey, String cDefault, String @cBuffer, ;
Integer nBufferSize, String cINIFile

Perhaps there are other APIs that I could/should be using. Can
someone do a better job of explaining them than what is in
the VFP help file?