Call waiting lets you handle multiple calls at one time. When you are on the phone, call waiting notifies you of an incoming call and lets you decide whether to answer the second call. Depending on your phone, call waiting shows the incoming caller's number or contact details on the screen or plays a tone.
You can answer the new call or send it to voicemail. If you answer the second call, the active call is placed on hold. You can switch between calls as needed.