@waterbear The video in the post you just replied to explains it better than words can but essentially it is:

when you create a block of content/text (not necessarily a sentence or a section header but a conceptual block that you define) the content is given a unique identifier. That content block can be re-used and embedded seamlessly fully-rendered in different documents (this is transclusion). If you update the block itself in any context, it is updated in all other contexts.