Sodium bicarbonate, the chemical name of baking soda, is an alkali, which reacts with the acid vinegar, to neutralize the 2 chemicals
Or, if you want to get technical.....
Sodium Bicarbonate (chemical NaHCO3) combines with the dilute Acetic Acid in vinegar, (chemical CH3COOH) to form Sodium Acetate, Water and Carbon Dioxide gas. The gas is released in the form of small bubbles. If the mixtures is strong enough and warm enough, it fizzes rapidly.
NaHCO3 + CH3COOH → CH3CO2Na + H2O + CO2 (gas)
In other words the salad dressing (vinegar) and the powdery stuff (baking soda) explode when they touch each other
Vinegar is acetic acid: CH3COOH
Baking soda is sodium bicarbonate: NaHCO3
Mixing the two is simply and acid base reaction. When an acid and a base are placed together, they react to neutralize the acid and base properties, producing a salt.
CH3COOH + NaHCO3 ---> CH3COONa + H2CO3
That last product is carbonic acid which quickly decomposes into
carbon dioxide and water:
H2CO3 ---> H2O + CO2
The CO2 is what you see foaming and bubbling in this reaction.