Similar to a Digital garden, a second brain is an extended web of ideas that is readily accessible to expand your thoughts and build your knowledge.

Typically, a second brain is private, and a Digital garden is public.

The above image is from this website.

Here is an interview video with him that dives into suggestions and helpful principles. I've skipped ahead ot the most relevant section:

Neocyborgs for a great overview.

Building a second brain course illustrated notes (Extremely in-depth!)