A backlink is a link to another page where the current page is mentioned. For example, when file A links to file B, then file B has a backlink to file A.
In an Obsidian workflow, you can use this principle to avoid doubly-linked lists. For example, you only have to link to another page from a MOC without the need for an explicit link from the page to the MOC. The link from the page to the MOC is visible inside a backlink.
The only place where doubly-linked lists can be of use is in index files, to have a quick way of traversing back and forth through your index.