Ogbono is the kernel of African bush mango, commonly dried and ground for cooking. In Nigerian and other West African dishes, it thickens soup and produces a characteristic stretchy or slippery texture. The kernels are different from the flesh of ordinary cultivated mangoes.
Ground ogbono is used with meats, fish, vegetables and seasonings according to the dish. It is not interchangeable with egusi, which comes from melon seeds and thickens in a different way.
Reference: Further reading.