Stimulus Threshold refers to the minimum intensity required from a stimulus to produce a response from a human or an animal. It is also the point where a human or an animal first senses and responds to a stimulus.
For example, imagine someone puts sand in the palm of your hand, one grain at a time. You may not notice the sand until 10 grains are placed in your hand. In this case, the Stimulus Threshold would be 10 grains of sand.