It's basically congealed semen. Piggies normally get rid of them themselves, but sometimes you may need to help. This thread contains some pictures:
http://www.theguineapigforum.co.uk/threads/sperm-rod.94652/
They can usually be gently removed from the protruding penis. Some baby oil and cotton buds can help with the job.
I've had several boars, and only one has had them regularly.
That's another term you'll never read in a guinea pig book - grease gland, anal sack and sperm rod!