Hypo (under) sensitivity means being under sensitive to the environment (for example, sound, touch and light, etc.). If you are under sensitive to something, that means your brain is not responding to the stimulus - even if your physical body is.
Some people with ASD experience hyposensitivity - although from what I've read, it appears that hyper (over) sensitivity may be more common.
Only after really thinking about this subject did I realise that hyposensitivity can be very dangerous. This article looks at why, gives some examples, and suggests some helpful strategies to deal with hyposensitivity.