Making an extended filter requires some understanding of logical terms. The view is similar to the one used in making target groups.
The buttons in the middle (IS, IS NOT, etc.) are used to set conditions amongst variable's categories, for instance when selecting (Q01=1) the = 'IS' . Another example using the command OR could be ((Ageband=1) OR (Ageband=2)).
The buttons at the bottom (And, Or) are used to combine different conditions defined using the commands in the column. In the example above, after defining (Q01=1) to connect with the second condition ((Ageband=1) OR (Ageband=2)), I need one of this bottom commands (in this case AND). So that I'll have (Q01=1) AND ((Ageband=1) OR (Ageband=2))