Hard to understand Elasticsearch optimization article

Elastic Stack Elasticsearch
1

I read the article about optimizing Elasticsearch table but I'm not able to understand what does author mean by following line.

These fields however aren’t actually “stored” in Elasticsearch, rather, they are only stored in the inverted index.

Can anyone please explain? It would be great if you can also explain about _source field.

2

When a field is stored, it means you can have the value of the field returned in a search result. If you don't store it then you will know that it exists in a search result, but you won't know what the result actually is.