In many parts of the U.S., soda is a sweet, fizzy drink. There are also some places where you'd call soda "pop," a "soft drink," or "Coke" instead.
While you may think of soda as something you order to drink at a restaurant, soda is also used for unflavored carbonated water, or seltzer, and for baking soda, or sodium bicarbonate. There's also sodium carbonate, or "washing soda," which is an ingredient in powdered soap and used to make some types of glass. This kind of soda was originally used to carbonate water, which led to the use of soda to mean "carbonated drink."