Tempeh is made from soybeans, like tofu. Unlike tofu it's made by fermentation into a kind of cake, and is made from whole beans so it has more fibre and vitamins than tofu, as well as a different texture. There are also a few different kinds of tempeh, which may include other ingredients like coconut, cassava or peanuts. At least one is made with leftovers from tofu production.
It's popular in Southeast Asia, where it's often fried (either with or without batter, or grilled on skewers. If you're trying tempeh, it's probably worth looking up some Javanese recipes to try.
✅ Pros: Nutritious and versatile. Keeps for a long time when cooked. A rare non-meat source of vitamin B12
❌ Cons: Made from soy and other things which may trigger allergies in some people