TLDR; mouse chording is triggering custom actions (like copying text) by holding down a mouse button and then pressing another. This is best done with a three-button-mouse, where the middle button does not double as scroll wheel.
I became aware of this when I discovered Acme, the text editor of plan9 - which uses mouse chording for a few things.
This explanation of mouse chording in Acme is pretty good.
I found it very interesting and wanted this to work system wide on my computer.
After some hacking in AutoHotKey I got the following setup to work:
Left + Middle = copy
Left + Right = paste
Right + Left = undo
Right + Middle = redo
Middle + Left = enter
Middle + Right = space
Middle + Scroll = switch app (Alt+Tab)
I find this very useful, and it saves me from needing to switch back and forth between the keyboard and mouse all the time, which is annoying.
I can also be productive while using only the mouse - especially for simple tasks like data entry; copying text fields, one by one, from one window to another.
To do this effectively, one really needs a three button mouse, often referred to as "CAD mouse" (because of some software called CAD that requires this).