Edges
The lines between the nodes and actions, are called edges (like any line in a graph). These edges indicate the direction of the flow within an activity. They are solid lines with open arrow heads.
You can see where the activity starts, goes to Action A, then Action B and then ends. If you cannot see where the arrows go, you probably should not be driving.
Consider this use case
This shows the basic flow of events for a Lecturer viewing and changing a student’s marks. I think this is pretty straightforward up to this point.
There might be times when your activity gets quite big and all over the place. For diagrammatic ease, you can split the edges using labels. A label is a circle with a letter in it. An edge basically goes “into” a label and comes “out” the corresponding label. This is shown in here where the edge comes from “Action A”, enters the label “a”, and comes out the other side (from label “a”) and goes to “Action B”.
Try to stay away from labels. They often cause more confusion and frustration than what they’re worth. See if you cannot rather rework your diagram to not need labels.