RotLatency tracks how fast you respond to cooldown expiration. It's purely informational and not meant to derive a "score" or anything.
You can display your averages either from the command line (/rotlatency stats) or via tooltip on a LibDatabroker display, such as Chocolate Bars.
Each ability/action is tracked separately and an overall average is displayed.
Configuration is done through Options->Interface->Addons or just click RotCooldown on your LDB display.
Configuration Help
"GCD Spell" is an ability that only uses the GCD -- i.e. it doesn't have its own cooldown. Find Herbs is an example.
Add a spell under "Spells To Track."
Check "Track GCD" if the ability only uses the GCD. In other words, the ability has no cooldown of its own.