Trays are the same as Sections, and have the same nested layout components (Rows, and so on). The main difference from Sections is that trays do not fall inside the layout structure below Posts. Trays only belong to the Site. This way you can add a nested Layout structure from the section level down on any place you want, not just inside a Post context. This is useful for footers and headers for example, which you want to have on every page of the Site, not just inside a certain Post.
Read more on how to use them inside your theme here.
You can create Trays on the Content Types page in the Site Dashboard, under the Trays tab.