The Hand of Nod is the signature structure of Nod. It's also its barracks.
It is perhaps one of Nod's most recognizable and distinct buildings, integrating the shape of a human hand into the architecture. This reflects the Brotherhood's use of symbolism. In this case, the infantry trained are the "hands" of Nod, changing the world according to Kane's will.
The building trains infantry and is also a prerequisite for other, more advanced structures.