A reactant is a substance that changes in a chemical reaction. Vinegar and baking soda are reactants — when you mix them together, they bubble up and make really good lava for a model volcano.
Reactants usually hang around the chemistry lab, but the concept isn't difficult if you think about the word react. When people react, they do something such as laugh at a joke or cry at sad news. In chemistry, reactants are part of something happening too: if two chemicals bond when mixed together, or if one dissolves, they are reactants. Any kind of chemical reaction involves reactants.