Backlinks, or bi-directional liks, are becoming table-stakes for productivity apps since they’ve been popularized by Roam. It’s a simple concept: when you create a link from page A to page B, page B will also reference page A. With traditional hyperlinks, page B wouldn’t know about page A unless you explicitly link it back.
Backlinks allow a graph of knowledge to grow organically. When you create a doc for Orders, and mention Products, a Products page will be created or updated with a backlink. Even when not actively documenting Products, readers can get an idea of what they entail because of the linked references.