Watch breakpoints (or watchpoints) are set on variables. They allow thread interruption on variable modifications.
What you need to do to add a watchpoint is :
Also, the new breakpoint appears in the Breakpoints View list.
While the breakpoint is enabled, thread execution suspends after variable value changed. The debugger selects the thread that has suspended and displays the thread's stack frames. The line where the breakpoint was set is highlighted in the editor in the Debug Perspective.