Targeting broadly essentially means that you're mostly relying on Facebook's delivery system to find the best people to show your ad to. This approach can lead to Facebook to find potential customers you never would've known about otherwise. Even though you still have to use at least a few basic targeting parameters, it's a good approach if you're not sure who you want to target.
With broad audience targeting, you can reach people who've expressed interest in your products (or products similar to yours), even if they haven't visited your website or app yet. When you target a broad audience, people in this audience automatically see products from your catalog that are likely relevant to them.
Requirements for broad audiences
Facebook pixel: You'll need a Facebook pixel implemented on your website, and use standard events to report which product IDs from your catalog are being viewed, added to cart and purchased. You may need to work with a website developer to set up your Facebook pixel and standard events.
Catalog: You'll need a catalog to use dynamic ads. If you don't have one yet, learn how to create a catalog. In many cases, you can use an existing feed from other online shopping portals with one of Facebook's third-party integrations.