Similar to Theme Forest or Wordprress store, it's an online directory of downloadable free Plone themes, maybe in the near future support paid themes. However, regarding to the number of free theme for Plone 5, there are a lot that are unaccounted for. I know most themes are customer based themes, while others are generic based on the market. I for instance have several themes that I reuse on various projects.I tend to do a lot of fancy Diazo work that is reusable in any project. For instance, auto-collapsing breadcrumbs diazo rule.
I think that the Theme Store will encourage more themes to be created and if there's a space to sell themes, then it will encourage designers and theme editors to create more themes.