The most useful CS topic I know of is object indirection - aka pointers (and pointers to pointers)

Once you realize that everything is stored at an address (or group of addresses) "somewhere", your life gets much easier :)

Whether it's memory addresses for a linked list, inodes for [local] file storage, or an IP address for a network resource, they're all "the same" in the sense of "this thing can be found by going to that place"

The world wide web is just a complex linked list/graph with loops

But if you don't grok address/reference indirection, your life is much harder