Anything that already brings out a response. Food, for example, makes a dog salivate.

This is whatever happens due to the unconditioned stimulus. When food is presented, the dog begins to salivate.

This is the new addition to the existing environment. Adding a bell to meal time will associate it with food.

The bell being added to meal time is the new stimulus. The bell is associated with food.

Now the dogs associate the bell with food. Therefore, the dog begins to salivate when the bell is rung.