System Lens is a Menu Bar utility that comes as an alternative to using Activity Monitor, a default Mac program, for checking your CPU resources usage. The application has simple to configure settings, uses minimum CPU resources while running on your status bar, and provides you with accurate info.
In the Preferences menu of the program you will be able to define Low, Medium, and High threshold limits. The running apps will appear on the main menu of the System Lens program only if they overcome one of the entered threshold limits. Also, you have access to a quick way to shut down any of the listed apps (there's an X button on right side of each item within the app's menu).
Another advantage is that you can select which programs to be ignored by the System Lens monitoring. Also, you don't have to pay a buck to use this utility on your Mac.
Personally, I think Activity Monitor, Mac's default tool, is far more helpful and feature-rich than System Lens. The default utility provides you with many advanced settings and info (CPU time, number of threads, etc.), offers you accurate energy and network details, etc. Hence, I'd say it's worth installing it on your Mac only if you're looking for a quick solution for checking CPU resources usage and quitting running programs.