This might be dumb, but what are Iridium Flares?

Peter Harding wrote:
In article ,
says...

What about the -9's?


Haven't managed to see one myself yet. Yet. I'll almost definitely have
to travel to track centre. Worth doing I s'pose! Next up is 5.21am on
Saturday ... well, it's only an hour before I'd be up anyway..
mmmmmmmmm... nice idea.

You can see the brighter flares in daylight also