Mosaic is an different approach to maintaining and displaying content in a Drupal website. In order to start explaining what Mosaic is all about, I'll try to explain how it works:
A pane is the surface on which all content is rendered on. It's basically a block that is converted to a grid.
A tile is one of the small rectangles rendered inside a panes. At this moment only text is rendered on tiles but in the future we'll add media, views, entities, menus and many more. Feel free to suggest in the issues queue.