The HP plus service is a printing software that can be activated on selected HP printers including 'e' model machines (those with an 'e' at the end of the model such as the HP LaserJet M234dwe).
Whilst there are some Pro's to having the HP plus service installed such as 3 months of free ink (with instant ink) and cloud based printing, the main con of having HP plus installed is that the printer will no longer be able to use compatible cartridges, and you will only be able to use original HP cartridges for the life of the printer.
It's worth bearing in mind that 'E' model printers such as the HP M234dwe have HP plus pre installed as part of the package. Once HP plus is installed, it cannot be deactivated.