Object caching is a common practice in software architectures to reduce database workload. In short, it is a copy of data from the first source and using it repeatedly.
For example, a website title fetches from a database only once and uses it three times on a page.
However, this copy destroyed when the page is fully displayed. This process will repeat again for the next visitor.
By default, this is how Object Caching in WordPress works. It only temporarily caches in PHP resource.
To make it reusable for the next visitor and across pages. The copy needs to store in other methods. This practice is what we called Persistent Object Cache.