Key Health Indicators (KHI) are Performance Counters with recommended thresholds aimed at revealing problems that can impact the user experience. They are really useful for ensuring the health of your Skype for Business Server environment.
Microsoft has recently updated version of the Key Health Indicators for Lync Server 2013, Skype for Business Server 2015 and Skype for Business 2019.
The download contains the KHI Guide, which outlines the operational process and remediation steps. A sample PowerShell script used to configure KHI Data Collectors. Lastly, the Analysis and Definitions Workbook which can analyse KHI performance data.
I strongly recommend you have these collected and reported on in your Skype for Business Server environment.