304 Not Modified is an HTTP server response code that is used to indicate that the webpage was not changed or modified since the last time it was crawled or loaded.
It lets the browser show a cached version of the post and when a crawler comes to the website it indicates that there is no need to crawl this page again because nothing has changed on it.
The importance of 3014 Not Modified in SEO of large websites is that Google will not recrawl the old and unmodified pages and will use the crawl budget to crawl new and updated pages.