Titanomachia literally means "battle of the gods". This was a turning point in Greek mythology. So the story goes something like this:
Uranus and Gaea (heaven and earth) have 12 kids. Of particular interest is one Cronus, their youngest son. He was given the responsibility of castrating his father by his mother, in order to separate heaven from earth. After this, he becomes King of the Titans. He and his sister Rhea give birth to several children (Hestia, Demeter, Hera, Hades, and Poseidon) all of which Cronus swallows, because he was told by his parents that one of his own children would overthrow him. Rhea hides Zeus from Cronus, and makes Cronus believe he has eaten Zeus (by wrapping a stone in cloths). When Zeus grew old enough, he confronted his father and forced him to regurgitate his brothers and sisters, then waged war on Cronus and the other titans. After 10 years of fighting this war (the Titanomachia ;), Zeus and his siblings imprisoned the titans in a cave beneath Tartarus.

Final note: this information was taken from a few (potentially not the best available sources) basically, other E2 nodes, and the Encyclopedia Britannica