Activity diagrams is a parametrised behaviour represented as coordinated flow of actions.
Activities can express the complete activity on a system, but often this will be impossible to cover all activities in one diagram. Therefore a number of diagrams is often produced, where each diagram expresses a small fraction of the activities in the system.
A system comprises of many activities the reflects the interaction form outside the system. Many of theses activities is important and contains dynamic functionality that can be expressed in a diagram before implementing. The diagrams gives the developer an overview of the different activities without tying the activities to specific parts of the system-to-be, e.g. HW blocks or SW modules.
This diagram expresses an activity of logging in to a system and create a token, that can be used in the rest of the system as authentication. The activity requires two parameters: Username and Password.