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