The “erg” is the colloquial term for ergometer, which technically refers to any exercise machine that measures the amount of work performed (I’m embarrassed to have explained this to a classicist, but others are reading, too). Today it is mostly used to refer to any exercise machine that purportedly simulates the experience of rowing a skiff on water—an indoor rower, so to speak.
All it does ultimately is to measure the wattage you generate by pulling on the handle as you slide back and forth on the rail to which your seat is attached. The combination of the flywheel and digital monitor allows the machine to convert your output to an estimate for a 500m split. So
robertito
above was essentially telling us that he was pulling about 20 times per minute (his seat rate), and, if he were rowing on water, he would be completing 500m intervals every 2 minute or so. There is also a neat (or silly) little mode on the monitor where you can track your virtual boat on a course and even put yourself up to compete against another boat with a preset split for 500m.
I think the modern versions of the erg were initially developed to train beginners and substitute for on-water rowing under inclement weather conditions. Nowadays, it is used for physiological training in a number of sports. In rowing, the scores are useful as a quasi-objective measure of physical fitness and, to a lesser extent, kinaesthetic aptitude for the sport.
I’m such a pedant.