Voice over Internet Protocol (VoIP) is what it sounds like: voice translated into data and transmitted across an Internet connection in the same way any other file or email is. When it reaches the other end, the data is transformed back into its original form and emerges like a regular sounding phone call.
Anyone, anywhere in the world can make phone calls using VoIP, as long as they have Internet access. No actual phone is required, not even a headset. At its most basic, VoIP works directly through the built-in mic and speaker on computers and tablets. Most email services, notably Google Gmail, include free video and voice, whilst Skype, Facebook, WhatsApp, Signal and a host of other apps allow voice calls over computers and cell phones to anywhere in the world. These calls transmit at no cost and without per minute charges accruing on the users cell phone plans. Kind of revolutionary if you remember the old prohibitive expense of calling long distance or internationally.