A more thorough explanation:

The Disclosure Document Program is a program offered by the U.S. Patent and Trademark Office that allows inventors to file a preliminary description of their invention and establish the date of conception before applying for a patent. This document can be used to establish a date of inventive effort for use in a later interference.

For example, let's say an inventor has an idea for a new type of phone case. They can submit a disclosure document to the U.S. Patent and Trademark Office to establish the date of conception of their idea. This can be helpful if someone else tries to patent a similar phone case later on, as the inventor can use their disclosure document to prove that they came up with the idea first.

The Disclosure Document Program is abbreviated as DDP and should not be confused with a provisional application under Patent Application.