Interlinking is the practice of writing posts that link out to other blogs and resources, so readers can move from site to site. It is the oldest way of making the blogosphere navigable: not just blogrolls and button walls, but link-rich text inside posts themselves, appearing in feed readers and giving regular readers new places to go. If the blogs you follow are not gateways to more blogs, that signals a weak link culture.
Adapted from Knitting the Web: We Need Interlinkers! on brennan.day.
In Praise of Links frames outward linking as the craft of surfacing connections: linking where a thing is discussed rather than just name-dropping. The point is not collecting linkdumps of uncritical bookmarks but curating the web for the reader, the way a blogroll curates a neighbourhood. See Being a Good Neighbour and Directories & Blogrolls.
Two long-standing metaphors describe what interlinking maintains:
Andy Matuschak's working notes take the principle to an extreme: evergreen notes densely linked, so the note system is itself a knit fabric. Related tools include Zettelkasten-style note systems.
The original interlinker was Justin Hall, whose Justin's Links from the Underground (1994) was a dense web of annotated links before the word “blog” existed. See History of Blogging.
Interlinking only works while the sites at both ends stay online. Links to dead sites rot, which is the problem described in The Long Web. When a linked site vanishes, the Wayback Machine snapshot is the standard fallback.