In the 2020's, this refers to writing down everything you know, usually in some graph structured way.
This is somewhat the centerpiece of Ciro Santilli's documentation superpowers: dumping your brain into text form, which he has been doing through Ciro Santilli's website.
This is also the closest one can get to immortality pre full blown transhumanism.
Ciro's still looking for the restore this plaintext backup on a new body though.
It is a good question, how much of your knowledge you would be able to give to others with text and images. It is likely almost all of it, except for coordination/signal processing tasks.
His passion for braindumping like this is a big motivation behind Ciro Santilli's OurBigBook.com work.
Cute synonym for second brain, sample usage: beepb00p.xyz/exobrain/#cntnt
zettelkasten.de/posts/overview/ mentions one page to rule them all:
How many Zettelkästen should I have? The answer is, most likely, only one for the duration of your life. But there are exceptions to this rule.