The contact sensor is an accessory you can attach to a Door or Window, it is installed on your Wi-Fi (like a lamp) in order to report its status to the Cloud and make it visible from the app, wherever you may be in the world.

The contact sensor reports when it gets opened or closed, and can create events.
It can also create events when batteries are low or when it gets tampered with.

In your Home Timeline, you can see those events listed, and if you have activated the setting (in User Settings -> notification Settings -> Monitoring), you can receive push notifications on your phone when those events are created.

The contact sensor is only available in certain countries, and to be installed in homes in those countries, so make sure your WiZ home Settings indicate it is located in the country where you purchased the contact sensor.