The 404-type layout in Beaver Themer is used to create a custom 404 page. This Themer layout is different from other types, in that it can only apply to a single page, so in the usual case you'll be creating static content rather than using field connections in your modules.
The layout has two rows with three Heading modules, a Separator module, and a Button module. Notice that the Heading field has text instead of the usual field connections in a Themer layout, again because this layout applies to only one page. Here's a screenshot of the initial layout:
You can certainly use field connections if you want to. For example, by using the Site Title field connection in your connectionm you'll ensure that if you change your website name it will automatically update on the 404 page.
If you ever want to revert to this default layout, click the Templates button when you're editing the Themer layout in Beaver Builder.