PPT Slide
Imagine yourself having to order a deck of playing cards: a typical solution would be to first order them in terms of suits, and then rank-order each suit. If there were two of you doing this, you could split the deck between you and both of you follow the above strategy, combining your partial solutions at the end, or one of you could sort by suit, and the other sort by order within suits, both of you working simultaneously.
Both of these scenarios are examples of the application of parallel processing to a particular task, and the reason for doing so is very simple: reduce the amount of time before achieving a solution. If you've ever played card games that use multiple decks, you've almost certainly engaged in parallel processing by having multiple people help with the tasks of collecting, sorting and shuffling all of the cards.