Concord grapes are "slip" grapes, so called because the snotty pulp easily slips out of the skin with one gentle squish. In case you are wondering if you can use those bland things called "red grapes" to make grape jelly, jam, or conserve, you can't. Those are technically grapes, but their resemblance to Concord grapes is so slight it's barely even worth mentioning that the two are even related. Stand under an arbor of Concord grapes and their fragrance will tell you all you need to know about the grape agribusiness.
Here are the grapes after, well, after the skins have been separated from the pulps, the pulps cooked down and the seeds removed (with a food mill), the skins and the pulps cooked together, the sugar added, the grapes further reduced, and finally, now, at the present moment of this picture, the addition of the chopped walnuts (which happened just after the zest and juice of one orange). If you have never had a pot of grape conserve boiling away in your kitchen, you can simulate the feeling by watching this video whilst simultaneously rubbing Smuckers grape jelly around the rims of your nostrils and onto your chest as if it is Icy Hot.