PostEaseThe short answer
Native content is content created specifically for a given platform's format, audience, and conventions, as opposed to a single asset published identically everywhere. A vertical video filmed for a specific platform's short-form feed, with that platform's typical pacing and style, is native content; the same video re-uploaded unchanged to a platform built around a different format is not.
Why native content tends to outperform the cross-posted version
A platform's ranking system, and its audience's expectations, are both tuned around content built for that specific context. A video that looks obviously repurposed from elsewhere tends to underperform one that feels native to where it is being watched, even when the underlying content is otherwise similar.
This is the core reasoning behind repurposing over cross-posting
Native content is the goal a good repurposing strategy works toward: reshaping one source asset into a native version for each platform, rather than distributing an identical copy everywhere.