Programming with parallelism is sometimes illustrated with a kitchen example. One cook working by himself has to do everything in sequence: turn on the stove, mix the batter, pour the sauce, grill the meat, etc. That’s serial programming with no parallelism. That's how all computers worked until fairly recently.
But two cooks can work faster than one. While one stirs the batter, the other can be checking on the meat or seasoning the broth. Ideally, the work gets done twice as fast. In practice, it’s not usually that perfectly efficient, but, even so, two heads are almost always better than one.
Taken to extremes, you might think that 20 cooks in the kitchen would work 20 times faster. A soufflé in 15 seconds! A medium steak in under a minute! But of course, that’s not how it really works. That many cooks will get in each other’s way, colliding and bumping and interfering with everything. Not to mention arguing over who gets to use the whisk. There’s such a thing as too much togetherness.