Introducing concurrency
Concurrency is whenever things are happening "at the same time" or in parallel. To give a non-code example, let's talk about managing my household. When I come home on a Friday night, I have a series of tasks: the kids need to eat, to shower, and to be brought to bed, the laundry needs to be folded and laundry needs to be put in the machine, and to be fair, a lot more, but this is enough to illustrate the example.
If I were to do this without being able to do multiple things at once, it would be a very tough night and get very late. I would first make dinner—put a pizza in the oven and wait next to it—feed the kids, shower them afterward, then bring them to bed, and then fold the laundry afterward, turn the machine on again, and wait until it's done. Luckily, I can multitask, so it looks more like this: I put the pizza in the oven, in the meantime, I turn the washing machine on and maybe fold a few pieces of laundry, then...