The lazy builder API in Drupal allows the creation of highly dynamic content within a render array without having to disable the cache for the entire render array or the page the content is attached to. What this means in real terms is that the initial render array can be cached quite heavily, but lazy builders allows for additional rendering to be done in after the initial rendering pass to generate content.
There are several reasons why lazy builders might be useful. The most useful reasons are to present some highly personalised or dynamic content to a user, but you might also want to offset the rendering of some slow code to give it some more fine grained caching that stores the outcome in a cache bucket.
Lazy builders work by using a normal render array, but instead of rendering the content you inject a lazy builder placeholder into the content. A callback method is used to tell Drupal how to inject the content into the placeholder. Then, at a later stage of the rendering process, Drupal will call the callback function and replace the placeholder with the actual content.
Read more here: https://www.hashbangcode.com/article/drupal-9-using-lazy-builders