Please explain what mince is, as in mincemeat pie or minced anything, is it a process or what?

Mince can mean a couple of things. For mincemeat pie it is different, you can buy jars of mincemeat - though it makes a sweet pie apparently, not a meat pie. Otherwise mince is sometimes used instead of ground, I would buy ground beef or ground pork etc, otherwise would buy minced pork etc.