The tray is a feature where you can merge posts to make it a single post 

On the dashboard search for the keyword, then just simply click on the Add to Tray button (as many videos you want).

Access the tray from the dashboard’s top right corner View Tray button, a popup screen will appear on the left side with the videos you added to tray.

Go to the bottom of the popup and click on Create Post button, it will redirect your merged content to the post editor, where you can make any changes if you want and then simply publish it like a normal post.