The jackfruit tree, like its cousin the fig, belongs to the Moraceae family and originated in India. One of its peculiarities is that it is cauliform, meaning the flowers grow directly on the branches or trunk.
The ti-jacque (there are numerous spellings) is the unripe fruit of the jackfruit tree, a common tree on Réunion Island, and a relative of the breadfruit tree. When ripe, it reaches an impressive size, its flesh turns yellow and sweet and it may be eaten as a real fruit (as a dessert).
For some recipes, it's necessary to find "ti-jacque battu" (in the local dialect, this means "chopped"): the product is sold in Reunion markets. You could chop it yourself, but it's a messy job. Do it outdoors and coat your hands with oil, so that you're not coated with the very sticky juice.