Global policies allow you to enforce corporate standards and centralize governance with common security and logging policies. And all of these without impacting API development. Global policies can be set up to be executed just before, or just after, each of your API assemblies. Thus it allows enforcement of common enterprise policies across all APIs.
The video below walks you through uploading and setting up a global policy. You can find the artifacts used in the demo here
You can read more on this on the knowledge center
After I posted this video, one of my friends had reached out for a sample for using Redact policy within the context of Global Policies. I created a sample here