The source code window is the main window and is always visible.
The source code window displays program source code. At the left of each source line is an "active area". It displays a pointer to indicate which source line the program currently executes, and it indicates on which source lines breakpoints have been set.
New breakpoints can be set by clicking into the active area with the left mouse button. An existing breakpoint can be enabled and disabled by clicking with the middle mouse button.
The tiny plus '+' between the "active area" and the source line can be clicked on. If you do so, the source line's assembler code will be displayed. The plus turns into a minus '-', which, if clicked, will hide the disassembled code.
Mostly, source code windows are opened automatically. To open a new source file manually, click the right mouse button and choose Open Source or choose File|Open Source.