A PPA is a Personal Package Archive, and is a method of distributing software to users, without requiring developers to undergo the full process of distribution in the main ubuntu repositories.
PPAs can be used to extend the available software in ubuntu to both programs that are not otherwise available in ubuntu, as well as to allow newer versions, such as beta programs, that have not yet undergone sufficient testing to be imported into the main archive.
Security
PPAs have not undergone the same process of validation as regular ubuntu packages. End users install PPAs at their own risk. Although each key is cryptographically signed, in order to confirm an uploader, keys are not matched to specific individuals, except via their "launchpad" accounts.
Subsequently, installing a PPA should be considered to be a low-security alternative as compared to the main repository, but marginally higher security than simply installing software at random from the internet. As part of adding a PPA, you trust the developer to not only install packages, but also to allow them to provide ongoing updates.