A baby monitor is usually a pair of one-way radios or "walkie-talkies". A microphone is left in the baby's room or with the baby and the receiving piece that can be kept with a parent or caretaker. This allows the caretaker to keep an ear out for an unhappy baby even when they aren't in the same place. The adult can go outside and garden, or to a different part of the house, like the kitchen to cook and do something that might normally interfere with hearing the baby. The monitor can also be used when someone in the house is sick and may need to call for assistance.
Nowadays baby monitors are available with many features. Some have a video camera, video recording, wi-fi capability, microphone, temperature sensor and heartbeat sensor.