What is absolute threshold?

In neuroscience and psychophysics, an absolute threshold was originally defined as the lowest level of a stimulus – light, sound, touch, etc. – that an organism could detect (Wiki).
Most of our senses have minimum thresholds in which we can perceive input (e.g. vision, sound, door). This is usually a function of how many sensor neutrons we have and there is some variation between different people. These thresholds were part of our evolutionary past in that the sensory threshold levels we currently have were useful and kept us alive (for the most part).