Microcontrollers are small, low power, inexpensive processors that control basic functions, for example the computer that controls a microwave oven. These processors are incredibly inexpensive (powerful ones cost under $10), but don't have nearly the power of a processor that would be found in a phone or laptop.
Machine learning models can be lightweight ways to implement complex computations that the processors would otherwise be incapable of. The use of machine learning models on small, local devices without unnecessary internet connections or overpowered hardware can increase privacy and efficiency of many household products.
https://www.tensorflow.org/lite/microcontrollers