When you are on a phone call and someone calls you, you will be alerted with an audio beep and visual alert of the incoming call. You can answer the second call while keeping your first call active by pressing the 'Talk' key on your phone or by following the instructions displayed on your phone. At this moment, you will be able to switch back and forth between the two calls in progress. Check your user guide for details.
There's no additional charge to use the Call Waiting feature, however you'll be charged standard rates for both calls while you're connected to both calls.