In Computing, we often refer to "platforms". For our purposes, this is synonymous with the Operating System. The major platforms today are Microsoft Windows, (Mac) OS X and GNU/Linux. So when we say that something runs on all major platforms, we mean it runs on these three OSs.
"Platform" can also be used in a more general sense, such as "distribution platform", meaning an application or website from which media, such as music, movies, games or other software is distributed, either freely, at a cost, or both. Such a platform could, but does not need to, utilize DRM.