A portfolio is a curated collection of evidence that tells the story of a learner's growth. The key word is curated — students select what goes in, based on criteria they understand. This is very different from a folder where you put everything, or a scrapbook of completed work.
The goal of a genuine portfolio is student ownership and reflection. When a student can look at a piece from Week 2 and a piece from Week 20 and articulate what changed and why, that's portfolio thinking. It develops metacognition, self-assessment, and ownership of learning — all of which are explicitly valued in the Australian Curriculum's general capabilities.