You can read more about Trays and how to create them here.
You can render a tray by using the tray tag, which renders the theme file:
Tray theme files
To use a tray, you need to create it in the Site Dashboard on the Content Types page. Then you can add a theme file in the
trays folder. The name of the Tray Content Type is the file name of the tray.
For example, if you created a tray 'Footer' (name: 'footer'), you can create the theme file
trays/footer.plate. Inside the theme file you can call the variable
footer to call the tray. Other than that, it works exactly like a Section theme file.