It can be installed by going to ReSharper → Extensions Manager and searching for "Cyclomatic Complexity".
Cyclomatic complexity is a metric intended to show the complexity of your code. It measures the number of paths through a code block - the more branches and conditional statements, the higher the metric. As the code gets more complex, the metric gets higher. A higher value is a good indicator that a method is getting too complex, and is becoming a maintainability risk, and should be refactored. You can read more about cyclomatic complexity at wikipedia.
The complexity of a method, function or other code member is always available as a tooltip on the method name:
When the complexity reaches a configurable threshold, the tooltip is shown as a warning:
And the complexity threshold can be configured in the options, per language: