A carboxylic acid is any molecule that has a COOH group. This group contains a carbonyl (C=O double bond) together with an hydroxyl group (OH) on the same carbon atom. Because it is easy to remove the proton with even a weak base, these compounds are called acids. An example of a carboxylic acid is Acetic acid, which is also known as vinegar.
Carboxylic acids are found a lot in food. Many types of fat molecules are actually carboxylic acids. For example, chocolate and coconuts have these acids. They are also used a lot in soaps and detergents.