Join is a pseudostate (see Abschnitt 20.11, „Pseudostate“) which joins two or more concurrent transitions into a single transition.
The incoming transitions should not have guards. However ArgoUML will not enforce this.
A join is represented on the diagram as a solid (black by default) horizontal bar.
This bar can be made vertical by selecting the join, and dragging with button 1 one of its corners.