Gina - There are many different kinds of self-watering devices. Some use simple wicks to transport water held in a reservoir below the pot. More sophisticated ones are set up with a special porous substrate below the soil and a water reservoir below that and a sensor that reveals when the water reservoir needs to be refilled for the top. There is another type that is based on a vacuum seal around a special sensor inserted into the soil at an appropriate depth.
There are other systems as well, but all of the better self-watering devices require proper set-up from the outset if they are to work properly. Sometimes the sensors fail for different reasons, so regular monitoring is still required. Their primary advantage is that they provide a reservoir that allows for a longer period of time between watering refills. That is helpful for people who travel a lot. But they are not fool or idiot-proof.
If you search inder "subirrigation devices" you can find lots more information.
