It creates a void self-closing tag.
The forward slash character, when placed at the end of a tag definition, causes Haml to treat it as being an empty (or void) element. Depending on the format, the tag will be rendered either without a closing tag (:html4 or :html5), or as a self-closing tag (:xhtml).
See the HAML documentation: http://haml.info/docs/yardoc/file.REFERENCE.html#empty-void-tags-