Gadolinium is a chemical element (atomic number 64). It is a flexible metal in its pure state, but in nature is only found combined with oxygen.
Its primary use is in alloys because a proportion of as little as 1% will improve the flexibility and oxygen resistence of many other metals.
Its primary use in medicine is in compounds used as a contrast agent in magnetic resonance imaging. Gadolinium is slightly magnetic and reacts well in magnetic fields. These compounds are non-toxic as they are removed by the kidneys before they interact with any of the body's other tissues. Gadolinium is also used in shielding for radiation emitting devices because it absorbs neutrons readily. It is used in many phospors (substances that convert radiation to light when exposed) that are used in other radiology procedures.
However, like many heavy metals, some gadolinium compounds are toxic. It has no known biological role.