The HTML Template Asset is one of the types of content that can comprise a BRJS application. There is more information about assets on About Bundlers.
HTML templates are used to build user interfaces. The HTML bundler enforces a namespacing convention, to ensure that someone working in a blade cannot accidently overwrite an HTML template in another blade. An error is reported if a template violates the namespacing convention.
The bundler verifies that the first top-level element in each file has an id attribute, and that the attribute complies with the namespacing convention.
For example, say you have an HTML template (a "View" in Presenter parlance) with a top level element like this:
<div id="novotrader.fx.tile.fx_tile"> ... </div>
We could tell that the name of the app it belongs to is novotrader, the bladeset is called fx and the blade is called tile.