what kind of thresholds you should change depends on your setup and what you deem to be acceptable. checkmk delivers “sane defaults” for the thresholds which should work for most systems. But of course these values can not work for every system, so you have the ability to change them.
For example you might have a system that is constantly at 90% RAM usage and this might be okay, because it does not change that much over time and the system is running stable. But this would be a topic regarding capacity planning.
Also these definitions are not directly “monitoring-related”, this is just the information the linux-kernel gives you regarding the memory-usage.
There is a great artictle directly from checkmk-blog diving into Linux-Memory-Monitoring i would recommend reading, i think it might help: Linux memory monitoring: RAM or Swap?