In most legged vertebrates, the lower leg consists of two bones, the tibia and the fibula. Having these two separate bones leads to increased stability and improved lower leg -- and therefor foot -- flexibility. In some species, the tibia and the fibula are fused into one bone, the tibiofibula. These are primarily species of frogs and toads, but also includes the tarsiers. Fusing these bones together results in a stronger, but less flexible, lower leg, useful if a significant portion of your survival depends on your ability to jump high.
'Tibiofibula' is also sometimes used to refer to the tibia and the fibula as a unit in humans, especially in cases where some pathology affects both. The adjective tibiofibular is more commonly used in these cases.