What is a "Blue printed" action?

[Shane Lindsey]

Afternoon all,
I have been reading around and see this term often when referring to an action.

Can someone please explain this term/process for me. Never heard it until I got on this website.

Thanks,
Shane

 

[NomadPilot]

Blueprinting for Accuracy of Custom Built Rifles

It's basically making everything square and straight.