A chemical reaction is when a substance (or a few substances) change into another substance. Chemical reactions are chemical transformations.
If you take a chemistry class, you'll definitely learn about chemical reactions: when one or more substances change into something else. When making beer, yeast turns the sugar into alcohol: that's a chemical reaction. When something iron rusts because of exposure to air, that's a chemical reaction. There are many types of chemical reactions, but they all involve transformations or changes. Water boiling — becoming a gas — is not a chemical reaction, since the steam is still water; the substance didn’t change.