Cacheability of render arrays
Short version in the API documentation: Render API overview: Caching
Bubbleable metadata
The parent page explains cacheability in general. This looks in detail at how render arrays bubble cacheability.
Auto-placeholdering
Drupal 8's Render API automatically creates placeholders of highly dynamic parts of a page, to have the best possible cacheability.
Render arrays
"Render Arrays" or "Renderable Arrays" are the building blocks of a Drupal page.
Render elements
Render elements (including form input elements) in Drupal
The Drupal 8 render pipeline
How Drupal renders pages