A platform in the context of software/services is a collection of related APIs/protocols/formats that can be used to build sites/apps, and sometimes misleadingly used to refer to apps themselves, or suites of apps, to exaggerate their sense of importance, value, or impact.
Operating systems are platforms.
apps are not platforms. Sometimes suites of apps are called a "platform" to exaggerate their sense of importance or as a form of superlative marketing/consultant-speak, when really they're just tools. E.g. "Communication platforms and software can be indispensable".
Then again, sometimes it's just the best word to use under certain cicrumstances: Always Own Your Platform
Online meeting platforms