Bitters are spirits (typically neutral and high-proof) infused with fruit, spices, leaves, bark, roots, and herbs—collectively known as botanicals. That means, fundamentally, bitters are alcohol infused with plant matter.
When getting started with the Old Fashioned, you should always use Angostura bitters. Angostura bitters are the ubiquitous benchmark aromatic bitters and always the default choice when no brand is specified.