The word smelt has several different meanings:

1. To smelt means to heat ore (rock containing metal) until the metal melts out of it. When iron ore is smelted in a furnace, pure iron separates from the surrounding rock. This process, which people discovered thousands of years ago, allowed civilizations to create metal tools, weapons, and eventually engines and machines. A smelter is both the person who does this work and the building where it happens.
2. Smelt is the past tense of smell, describing when you detected an odor with your nose. If you walked into the kitchen yesterday and noticed the aroma of cookies baking, you smelt them. If your dog discovered where you hid its treats last week, it probably smelt them through the container.
3. A smelt is a small, silvery fish that lives in cold waters. Smelts are usually about the size of a pencil and travel in large groups called schools.