In full page caching, the complete page of the website is stored in a cache, and for the next requests, this cached page is served. Full page cache is more effective if your website content does not change too often; for example, on a blog with simple posts, new posts are added on a weekly basis. In this case, the cache can be cleared after new posts are added.
What if you have a website that has pages with dynamic parts, such as an e-commerce website? In this case, a complete page caching will create problems because the page is always different for each request; as a user is logged in, he/she may add products to the shopping cart and so on. In this case, using full page caching may not be that easy.
Most popular platforms provide either built-in support for full page cache or through plugins and modules. In this case, the plugin or module takes care of the dynamic blocks of the page for each request.