A functional group is an atom or group of atoms in an organic molecule that decides how the molecule reacts. Organic compounds with the same functional group react in a similar, general way, and this is why we sort them into families.
Alkenes contain a C=C double bond. Every alkene takes part in addition reactions at the double bond and decolourises bromine water, which turns from orange to colourless. This is true for ethene and for much larger alkenes. Alcohols contain the -OH group. Every alcohol burns in oxygen. Carboxylic acids contain the -COOH group, and every carboxylic acid reacts with a carbonate to give carbon dioxide gas.
This means that once you know how one member of a family reacts, you can predict how the others will react, because they share the same functional group. Propene decolourises bromine water in just the same way as ethene.
