Literally "card index", in ze german.

Roam and them have looked into this a good bit. Easy to find a deep dive on that. Like Andy Matuschak's working notes for which Andy built an interface where multiple notes can visible at once and with a hover state for peeking at the summary of a note. Andy argues for what he calls [Evergreen Notes]

See also [Digital Garden], [Foam], and [Commonplace].