There are various types of ToF sensors, but typically, Time of Flight measures how long it takes an object, particle or wave to travel a certain distance, reflecting off an object and returning to the sensor itself.
This actually depends on the optics, emitter and receiver types, as well as lensing and more. Generally, ToF cameras are able to accurately detect objects in a short distance of approximately 1-2 cm, all the way to many meters. ToF sensing applications are flexible and able to adapt to objects of different shapes and sizes.