The astragalus is one of two major bones in the archosaurian ankle (and indeed the ankle is rather important when it comes to archosaur phylogenetics). The astragalus and calcaneum are generally two fairly small and blocky elements that lie between the tibia and fibula and the metatarsals. In the theropods, the astragalus has an ascending process – a flat, triangular plate of bone that sits in front of the tibia and actually sets into a recess of the anterior face of the tibia. (This is also seen in basal saurischians, but it’s not as exaggerated as in the theropods). In some cases it can be really big, reaching a considerable distance up the face of the tibia (as I recall, up to about a third of the way up).
This is a nice example, a Tyrannosaurs astragalus with the subtriangular ascending process clearly visible and set into (and indeed fused to) the tibia. This is one of those classic little quirks of anatomy that, if noting else, provide and excellent little systematic feature – if you find even a small fraction of a skeleton out in the field and it has this, then you know you have a theropod on your hands.