The dictionary definition of a broadside is vague and indeterminate. However, this is just as accurate as there is no single sentence that could define the purpose or presentation of what a broadside should be or could be.
A leaflet, handbill, pamphlet, etc.
A broadside is a form of street literature, public art, a shared experience. It is meant to be read in public, creating a communal reading experience, rather than behind the closed doors of a library, a private reading experience. Share it with a friend or even a stranger if you can. The broadside is public and open (literally impossible to close).
As diverse as it is in form, it can be equally diverse in purpose as well. Historically serving to educate, instigate movements that are social, literary, and political; broadsides can disseminate information and ideas on a large, public scale.