It distorts gravity's pull on objects, allowing it to be practically weightless. In a ring-sized micro generator it has the power to lift and toss full-sized SUVs, as well as an eighteen-wheeler semi-truck. With a larger mass in a full-scale power generator, it attracts anything near it to its core, and is stated as capable of sinking the entire island they were on.
Physicist Dr. Franklin Hall devoted his life's study to studying Gravitonium. When he powers the generator in order to take down Ian Quinn, as well as many innocent people on the island, Coulson stops him and Dr. Hall falls into it. At the end of "The Asset", it is hinted Hall is not only still alive, but may later become the Marvel villain known as Graviton.