A more thorough explanation:

An addendum is an addition to a finished document, such as a contract. It is like adding a new piece of information to something that has already been completed. The most common type of addendum is an attachment or exhibit at the end of a document. This means that there is an extra page or pages added to the end of the original document.

For example, let's say you signed a contract to buy a car, but you forgot to include the color of the car. You can add an addendum to the contract that says the car will be red. This modifies the original document, so it should be signed or executed with the same formalities, unless the original document states otherwise.

It's important to note that an addendum is not the same as an amendment. An amendment changes the main terms of the document, while an addendum just adds something extra.

In some countries, like those that follow civil law systems, addendum means any modification of a contract. This means that any change to the contract, like extending the terms or changing the payment currency, would be considered an addendum.