We are going to start our explaination by giving two simple examples.
After eating all you can, you toss your chicken leg bone out of the car window (shame on you for trashing up the highway), but in short order an ant fnds your tossed chicken bone. One single ant could bite of the left over on the bone and transport it to the colony, one bite at a time; but, it might take him 1 whole day (24 hours) of work. But, what if he gets help? He signals some buddies and being a small colony of ants they allocate a total of 10 ants to do the task. Ten times the workers take one tenth the time. The ten ants do the task in 2 hours and 24 minutes.
I toss another bone out the window. An ant fnds it and the colony allocates 50 ants to do the task of picking the bone clean. In less than 30 minutes (28.8 to be exact) the 50 ants working in parallel complete the task.
One painter might take 8 hours to paint the exterior of an average sized house. But, if he can put a crew of 10 painters working simultaneously (or in other words in parallel) it takes only 48 munities. What about a crew of 50 painters assuming that they can do work and not get in the way of each other; well how about less than 10 minutes (9.6 to be exact).
Now let's make sure we understand that the same amount of work was done in the examples given. The work was only completed in a shorter amount of time because we put more workers on the task. Not all tasks can be divided up in this way, but when it can be divided between multiple workers, we can take advantage of the workers doing their sub part of the task in parallel. Let's look at another example.
I want to drive from Houston, Texas to Dallas, Texas; a distance of about 250 miles. For easy calculations let's say I can travel 50 miles in one hour. It would take me 5 hours. Well, I could divide the task between 5 cars and have each car travel 50 miles and arrive in Dallas in 1 hour. Right?
Well, wrong. The task of driving from Houston to Dallas cannot be divided into tasks that can be done in parallel. The task can only be done by one person driving in a line from Houston to Dallas in 5 hours. I used the word "line" because it helps connect us to the word: linear. A linear task cannot be broken-up into smaller tasks to be done in parallel by multiple workers. Within the computer world, the word associated with linear concept is sequential processing. I must drive one mile at a time in sequence to get to Dallas.
Our natural tendency is to share the work that is to work in parallel whenever it is possible. As a group we can accomplish many tasks that can be done in parallel in less time.