What it is
A giant tropical fruit; young green jackfruit shreds like meat and has been a South-Indian and Southeast-Asian curry staple ("kathal") for centuries — the "meat substitute" framing is just modern marketing on a traditional food.
Why your biology objects
A whole fruit — green jackfruit is mostly starch and fibre, ripe is rich in potassium, vitamin C and carotenoids. The one thing to know: as a meat stand-in it's a starchy vegetable, not a protein, so pair it accordingly.