Transitional fossils are the connection between two species of organisms that shows the relation between the two species.
They are missing links that show the evolution of species.
Archaeopteryx is considered the missing link between the dinosaur and birds. They are primitive birds. They show both the characteristics of dinosaurs as their skeleton is the same as dinosaurs but also has flight feathers.
Australopithecus shows the link between the apes and the humans. It has some characteristics of apes like a small brain but also some features of modern humans like bipedal.
Hence, the transitional fossils help us to understand the better concept of evolution.