Jackfruit is a large fruit native to Southeast Asia. These things are enormous and can weigh up to 80 pounds!
The reason jackfruit comes up for vegans is that the unripe jackfruit can be used as a meat substitute. It is used in meals like curries and BBQ dishes.
Don’t worry, you don’t have to hunt down this curious fruit at your local supermarket too cook with it (it won’t be there). Companies package up and flavor jackfruit so you can simply add it to your dishes. If your local natural grocer or Whole Foods carries this, it will be found by the tempeh, seitan and tofu.