When you break on a particular line of code, execution stops whenever that line of code is executed.
To set a breakpoint on a particular line:
In the Source window, select the line and press F9.
Alternatively, you can double-click a line.
When a breakpoint is set on a line number, the breakpoint is shown in the Source window by an asterisk in the left column. If the Breakpoints window is open, the method and line number is displayed in the list of breakpoints.
You can toggle the breakpoint on and off by repeatedly double-clicking or pressing F9.