Scarcity meaning that if person A owns it, person B/C/D etc. cannot, because (in this example) there is only one. Most digital files are infinitely copyable, and there aren't good DRM solutions for managing this conundrum that actually give real ownership rights to the owner... until blockchain.
Why is this a problem? Ask a creative content creator.
How is blockchain solving this in practice? One example: https://www.niftygateway.com/