l have been watching stills and videos of climbing and bouldering.These people are super-athletes with an apparent death wish :-)The verb ''to flash'' seems to indicate some positive aspect in climbing.To what does this refer?

To flash something in climbing is when you successfully reach the top of the climb or boulder problem on your first go but you had beta about the climb. Beta may consit of watching someone else climb it or someone telling you about the climb, or reading about it in a guide book.
The only thing better than flashing a route is onsighting a route which is when you reach the top without knowing anything about the climb and without resting or falling.