Those script tags are a common way to implement templating functionality (like in PHP) but on the client side.
By setting the type to “text/template”, it’s not a script that the browser can understand, and so the browser will simply ignore it. This allows you to put anything in there, which can then be extracted later and used by a templating library to generate HTML snippets.
Backbone doesn’t force you to use any particular templating library – there are quite a few out there: Mustache, Haml, Eco,Google Closure template, and so on (the one used in the example you linked to is underscore.js). These will use their own syntax for you to write within those script tags.