That happens to me all the time: I read and read and read about some topic I want to understand and it all just seems to remain out of focus. And then I stumble upon a sentence, analogy, metaphor, or diagram and it all suddenly clicks into place. I'm happy to hear this did it for you, Tara :-)
To explain what an index is let me use a map analogy.
Imagine you've got a map of Spain. That's your Zettelkasten. It's a highly detailed map where you can see all the roads and all the cities and towns and villages. Each city/town/village is a note for an idea and the roads are the links between those ideas.
With that map in theory you have all the information you need as a tourist to plan a road trip. However, the info is not entirely in a convinient form. You wouldn't know where to get started with your road trip and which cities to visit--we assume you don't have time to visit every single city in Spain. What you need is some additional information. Something that tells you which cities to prioritize depending on what you're interested in. That's an index.
For instance, if you want to travel around Spain and you're mostly interested in beaches, you'd need an index of the cities were the best beaches are. If, on the other hand, you're mostly interested in culture, you might want an index of the cities were the best museums and historical buildings are. Or if you're mostly interested in food, you'd need an index of the top culinary cities.
So you can think of an index simply as a list that gathers together the top notes that fall under a common theme.
As to how to implement it, it's just a list with links (a.k.a. pointers or references). Here's an example:
Title: cities of Spain with great food.
Links:
- Barcelona: [link to note about Barcelona]
- Madrid: [link to note about Madrid]
- Pamplona: [link to note about Pamplona]
- Bilbao: [link to note about Bilbao]
How exactly you implement the links depends on the tool you're using. In my case those links would be filenames with a timestamp as a prefix, all surrounded by double square brackets, e.g., something like this:
Title: cities of Spain with great food.
Links:
- Barcelona: [[201811091101-Barcelona]]
- Madrid: [[201710080931-Madrid]]
- Pamplona: [[202001022119-Pamplona]]
- Bilbao: [[201512291331-Bilbao]]
I hope this helps!
