14 DAY TRIAL // The Ubuntu developers have finally merged the Plymouth package from Debian, and they are preparing for some important changes.
Plymouth is not something all that famous for regular users, and it's not even visible anymore for people who have SSD drives. This is an application that's responsible for the "splash" screen that you can see during the Ubuntu booting process. It's responsible for more than just a pretty image and it's also themable, although it's not usually advisable messing with it.
For some unknown reason, probably related to dependencies and such, the Plymouth component hasn't been updated from Debian with the rest of the packages since 2009. That's a very long time, but Ubuntu developers are making a push for Ubuntu 16.04 LTS, and they are preparing a lot of important upgrades. Plymouth is just one of them.
Plymouth is complex
Plymouth is more than just a simple and nice theme for Debian-based systems, it's actually doing a lot more than that. This is from Ubuntu's website.
"Plymouth is the application which provides the graphical 'splash' screen when booting and shutting down an Ubuntu system. Note that on Ubuntu, Plymouth is considered to be the 'owner' of the console device (/dev/console) so no application should attempt to modify terminal attributes for this device at boot or shutdown."
Plymouth makes use of KMS (Kernel Mode Setting) where possible (not Nvidia obviously), supports scripts and runs at system startup and system shutdown. It wasn't a priority until now to update it, but it's likely the team wants to do something nice and animated to the booting process, and they need a newer version of Plymouth.
Of course, if you have a fast SSD drive, you won't be able to see what's happening, but that's not of importance. You can check the logs to see all the changes and improvements that have landed after the Debian merge.