Parallel processing is the ability of the brain to do many things (aka, processes) at once. For example, when a person sees an object, they don’t see just one thing, but rather many different aspects that together help the person identify the object as a whole.
For example, you may see the colors red, black, and silver. These colors alone may not mean too much, but if you also see shapes such as rectangles, circles, and curved shapes, your brain may perceive all the elements simultaneously, put them together and identify it as a car. Note that motion and depth of the object can also be perceived. These cues processed in the brain tell the person that the red car is headed straight at them so they jump out of the way. Without parallel processing, the brain would have to process each aspect of the car separately in progression. By the time the person identified the car, it would be too late.