K2 SmartActions are a new feature coming with the next K2 update called 1420. K2 Smart actions enable workflow users to action a task without having to use a form.
It is a messaging based method of actioning work items using Microsoft Exchange Server. Email is supported out-of-the-box with options for alternative methods of cutom messaging.
Install time you setup options relating to this and off you go, Your K2 task notifications can now configured to access this additional capability.