Fruit salt is marketed under the brand name Eno.
It has a similar effect to baking soda. It causes flours to rise and is used as a leavening agent in many recipes in this book. Naan, dhokla and idli batter all use fruit salt.
Baking soda is sodium bicarbonate. When baking soda contacts moisture and an acidic ingredient, the resulting chemical reaction produces carbon dioxide. This expands with heat, causing the mixture to expand or rise. This reaction begins immediately after mixing the ingredients.
Sodium bicarbonate only reacts when activated with acids like buttermilk, yogurt, lemon juice or vinegar.
Eno, or generically fruit salt, is a combination of baking soda, citric acid and sodas. It has 46% sodium bicarbonate, 44% citric acid and 10% sodium carbonate. Fruit salt does not require the addition of acid ingredients.
Despite its name, it contains no sodium chloride, or common salt.
The best source of Eno is a pharmacist. Whilst it is available in supermarkets and Indian grocers, they tend to stock flavoured varieties, such as lemon. For cooking purposes you want to get the unflavoured variety.
Eno can be substituted by baking soda, provided there is sufficient acid content in the mixture you are using.