Static content is the files that stay the same for a long period of time. They are not subject to regular changes. So if you refresh the page, the same content will come up.
For example: Website logo, header image, social media icons, HTML files, product images etc..
We do not need to fetch these resources directly from the web server. As they are not changing regularly, we can make a copy of them near the user. So when the next time they load the page, those resources could be fetched from the nearest location. In general, it is called caching.
The caching reduced the latency and speed up the Magento store extensively. That’s why you experience website loading faster on the second round of loading with comparison to the first time.
On the other hand, dynamic content is the content that changes rapidly. So it has to be fetched from the server every time. For example, the live score in a cricket match. The numbers on the screen change regularly. It is not advisable to cache the dynamic content.