Kangkong (Tagalog) or tangkong (Aklanon) is referred to in English as swamp cabbage or water spinach although it's not really cabbage or spinach. It's rather ubiquitous in the Philippines, it grows everywhere, in ditches, in ponds, on dry land, or in cultivation.
Kangkong has a mild, not bitter flavor. The leaves are harvested before the plants flower to preserve the fresh, sweet taste. In Vietnam the green is eaten raw in salad or added to soup. In other parts of Asia the greens are lightly cooked with a savory or spicy paste made with chiles. The stems take a bit longer to cook so they should be added first then the leaves only take a few seconds to heat and wilt.
This is not a common vegetable in the U.S. Look for it in ethnic markets or buy kangkong seed and grow your own.