Bi-directional links establish a two-way connection between pages. This enables you to browse in either direction: follow a link to its destination, but then also browse backlinks, which are all the pages that link to the current page.

In practice, if you copy the link of a Slack thread to the Notion document, there is only a one-way link from Notion to Slack. If the link also enables you to navigate from Slack to Notion, it is a bi-directional link.