Package managers make life on Linux a whole lot easier. Instead of managing bits of software by yourself and sorting out the inevitable dependency hell, where one package depends upon another and that depends upon yet another and so on, you can have a clever bit of software do all the work.
Debian, Ubuntu and other derivatives use the .deb package format and Aptitude (Apt for short) as the package manager.
However, there are some occasions when your package manager can hinder, rather than help. It may end up insisting on installing or upgrading something you don’t want, or upgrading something you want left alone at its current version. Its persistence could extend to your update manager on your desktop nagging you every five minutes - wouldn’t it be nice to shut it up?