"What is a Primary?" You Ask

This one's for you, Mr  Rumcove!

Rumcove plaintively asked  during a telephone chat last week, "What the fuck's a primary?" Where to begin?  I told him I'd get back to him.

Luckily, BBC News has published What is a primary?  at its website here.  Here's a 1 minute, 10 second video report for anyone interested who doesn't understand the process.