This is a tough question to answer with a short number of words, because it's a complex issue, but here goes an attempt to keep it brief:
We interact with the world via our seven senses - sight, smell, taste, hearing, touch, vestibular and proprioceptive awareness. (I will give more details about these in other articles).
This sensory input is vital for survival - it gives the information upon which we decide on fight or flight. It tells us if we're too cold or hot, when we're being loud, if someone has just touched us in a caring or savage way. A healthy sensory processing system does this effectively.
Someone with Sensory Integration Dysfunction can have a few challenging things happening instead.
- overload - too much sensory information going in for the brain to tolerate. Usually represented by a meltdown.
- mistranslation - light touch can be painful, broken bones might go unnoticed, talking loudly can be misunderstood as angry yelling and in some less common cases the senses get totally jumbled (e.g.: sounds might have colour!)
- difficulties filtering sensory info - someone with auditory sensitivities in a classroom will hear the cacophony of these things: the fan on the ceiling, the clock ticking on the wall, the teacher booming, Suzie whispering to Kate, paper rustling, pens and pencil scratching, the terrible sound of textas, the chattering from three classrooms down, the birds tweeting outside, and any number of other sounds. Blocking out the unnecessary noises comes naturally to the rest of us because our brain's filtering system works, but it's impossible for this person. So focusing on just one of the sounds as more important than the others (such as the teacher's voice) requires a huge conscious (exhausting) effort of concentration. (Imagine doing that all day at school - now imagine the same degree of bombardment of information through the other senses at the same time!)
- modulation problems - hyper- and hypo-sensitivities to sensory stimuli. Sometimes these will flip back and forth without notice, seriously affecting behaviour as the individual seeks or avoids various sensory input in an attempt to manually modulate.
- anxiety - it's almost impossible to avoid any anxiety when living with SID, and usually the more anxiety an individual has to manage, the more difficult it is to deal with SID (and vice versa). Teaching anxiety management techniques is imperative, as the two things necessarily interact.