Cadmium is a reducing agent (something that can oxidize).
The terminology can be very confusing at first, but an oxidizing agent is called this because it has the ability to cause another substance to oxidize. To do this, it must be reduced at the same time.
Likewise, a reducing agent will cause the reduction of another substance, but it must oxidize in order to do so.
In the above half-reaction, cadmium is seen to release two electrons (and become a cadmium ion). Loss of electrons by a substance is always oxidation of that substance, and since a substance that oxidizes is called a reducing agent, cadmium is a reducing agent (which also means that cadmium ion is an oxidizing agent.)