TL;DR
Digital Twins are a mechanism for simplifying IoT solution development.

For Eclipse Ditto the Digital Twin is a concept for abstracting a real world asset/device with all capabilities and aspects including its digital representation.

A Digital Twin

  • mirrors physical assets/devices
  • provides aspects+services around devices
  • keeps real and digital worlds in sync
  • can be applied in both industrial and consumer-centric IoT scenarios

Industrial context

In the IIoT the Digital Twin metaphor is becoming a popular concept for tracking a produced product/good in its complete lifecycle.