If you want to carry out some variable operations, like moving a variable, or viewing its properties, you first need to select it.

You can do this by clicking on the green square at the beginning of each row. This selects the entire row. Notice how the icons on the Variables tab change to indicate they are now available.

You can select multiple variables in one go. Select the first one normally. Then, while holding down the Ctrl (Control) key, select any additional variables. Alternatively, hold down the Shift key to select all variables from the first one selected to the current one.