Lists is a relationship concept in tiddlywiki (https://tiddlywiki.com/#Introduction%20to%20Lists). Basically, instead of a child declaring who's its parent, the parent declares the list of its children. This allows for straightforward multi-parenting linking with custom sort order wherever required (tiddlywiki allows to change the sorting on query).
It's a simple concept that's extremely useful in practice in TW specifically because the wiki is built around it and almost every relation is defined within the same framework of lists.