It is the amount of free RAM that contains system cache data. The Task Manager also shows this.
That RAM can be discarded and used whenever an app needs it, so it isn’t decreasing available RAM. It is just the system taking full advantage of unused RAM.
The OS tries to keep it populated with recently and frequently accessed files, among other things.
Users want their RAM to be fully utilized since it is the fastest storage on your machine.
That said, SmartTrim now has some capabilities to periodically clear the cache, but it isn’t generally advisable.