The watched expressions window is opened using View|Watched Expressions. It displays arbitrary expressions.
To add an expression, type it into the edit field and press Enter or click Add. To remove an expression, click on it (choose the root of the expression) and click Del.
You can also move a variable or structure member from the local variables window to this window using the context menu in the local variables window.
The values of most expressions can be changed. For this purpose, press F2 while the input focus is in the window. Then edit the value and hit Enter. Note that you cannot modify the strings that char* values point to in this way, just the pointer value.
Watched expressions are stored across debugging sessions. It is recommended that you remove expressions that your don't need any longer because that speeds up the debugging process.