Dealing with more than one Process at a time.
Analogy
You have a to-do list for the rest of the day. Todo: Prepare and eat dinner, do laundry, study, talk on the phone Would you do this serially? No! That would be a horrible misallocation of your time. Most likely, you might start your load of laundry, then go and start to make dinner, probably start your call on the phone, eat your dinner whilst still on the phone and while your laundry is still going. Finish your food, hang up somewhat later after that, and begin studying. Cycle your laundry because it has finished washing, and then go back to studying. Finish the laundry later, and then go back to studying. Still is exactly how multitasking might work in a computer!
This is handled through Scheduling.