Interpreting Nagios check_disk output

This is "Performance data". Performance data is defined by Nagios as "everything after the | of the plugin output". This data is used by 3rd party programs for charts. Please see more details in the documentation https://nagios-plugins.org/doc/guidelines.html#AEN200

Any data after the pipe (|) in a Nagios check output is performance data, or "perfdata", which you can collect and process for time-series trending purposes (a Google for "nagios rrd" will provide a variety of tools to help you in this endeavour). What exact values are provided vary by plugin, and unfortunately the detailed help output from plugins isn't always the most useful. However, the set of semicolon-separated values is (supposed to be) standardised, as per the Nagios plugin development guidelines. Specifically, the values in your perfdata are:

• space currently in use
• warning threshold (the value of the first element that will cause a warning alert)
• critical threshold (as above, but for a critical alert)
• The minimum possible of the disk (how low the first element could concievably go -- '0' is always going to be the right answer here)
• Total capacity of the disk (the maximum that the first element could concievably go)

The first item is useful for drawing graphs directly; the rest are useful for drawing threshold lines (warn/crit) and scaling the graphs appropriately.