In macOS, a Text Clipping is a selection of text that you’ve dragged from an application to another location on your Mac, where it becomes a unique kind of standalone file.
The relatively little-known feature has been around since at least Mac OS 9, and it offers a convenient way to save out pieces of text from pretty much anywhere for later use in another app or document.
To create a Text Clipping, simply highlight any piece of text, then hold left-click and drag it with your mouse to your Desktop or an open Finder window.
Read more at MacRumors.com