Hi @Ziemowit - welcome.
Great question. I think the answer is ‘something that showcases your skills, whatever they may be’. I think describing your process is a key factor. The reason I put LaiYee’s portfolio in first place in that article is because she clearly shows her process, from concept to deliverable. UX is about the process as much as the final outcome, so rather than spending too much time putting together a slick collection of imagery (although that never hurts, of course), I’d focus on collecting documentation of what you did along the way.
If you don’t have a huge number of projects under your belt because you’re just starting out, you can use personal projects to demonstrate your skills just as successfully. Or consider offering your services for free to a school or not-for-profit.
I hope that helps.