You'll find the pins move. In one position the they will turn/keep the current on - in the other they will turn/keep it off.
You'll need to experiment to find out which position is which.
Some of the fancier ones also have a button you can press to temporarily override the decisiion made by the pins
The face represents 24 hours and each pin all the way round represents either 15mins/30min or 1 hour. You then push in or pull out the pins as to when you want the timer to turn on or off. If you want to say turn a lamp on at night when you are away from home the pin marked around 1800 hours - approx 6pm those need to be pushed in or out depending on the model you have. This would make your lamp turn on at night when you would normally get home. On mine you click the pins inwards to represent 'light on' and pull them out for 'light off'.
When I'm away I will switch the light to come on at the approx time I usually get home and then off when I go the bed usually. But then during the evening I will turn in on and off a couple of times to make it look like someones at home.
Just to confirm if you wanted the timer to switch on between 6pm and 9pm then you have to push ALL the pins inwards between 1800 hrs and 2100 hrs. If you were to push in just one pin at each time interval the timer would just turn on a 6pm for a few minutes and then turn off and then on again at 9pm for a couple off minutes and off again.