“Content” means that it is a deployable project item, it signals that the file needs to be copied to the target machine.
Something you can see with a simple console mode project. Project + Add New Item, pick the Bitmap File item template. Its Build Action is automatically set to “Content”. Use Project + Properties, Publish tab and click the Application Files button. Note how the bitmap automatically got added to the list of deployed files:
Go back to the Properties window and change its Build Action to None. Click the button again and note how the file is now no longer included.
Also used by installer utilities that integrate with VS for the exact same reason. And a big deal in web applications, they usually have a lot of files that need to be deployed to the web server.