Sensory processing refers to the way the nervous system receives messages from the senses and turns them into appropriate motor and behavioral responses.
Sensory Processing Disorder (SPD) is a condition that exists when sensory signals don't get organized into appropriate responses.
This website provides much more information
Many children struggle with sensory processing. This can be anything from getting upset if their hands are dirty to being too sensitive to tags in clothing to chewing on things to craving constant motion. Children from hard places are more prone to sensory processing issues, but all children can benefit from having some sensory activities throughout their day.
Here's one super easy activity I set up for my preschooler (Yes, she is sitting ON the table, that happens a lot, and is often not a battle I choose to fight!):
I used one of those plastic shoebox type bins and put some soft, colorful pom-poms, clothes pins, and a plastic ice cube tray in the bin.
My 4 year old loves to pick up the pom-poms with the clothes pins and that is really good for her fine motor skills that will help with handwriting later. Also, the soft texture of the pom-pom balls is a good way to exercise her tactile processing. The repetitive nature of the activity is also calming for an overstimulated child who needs to settle down. And as if all that stuff wasn't enough benefit from this easy activity box, kids love to sort the pom-poms all kinds of different ways into the ice cube tray and this helps with mathematical skills and logic thinking.
Also, super awesome, this activity might just entertain your preschooler long enough for you to shower in peace!