An operating system can use many different scheduling algorithms to manage processes running at the same time. The scheduling algorithms will determine the way the operating system allocates CPU time for each process. The CPU is the central processing unit, the processor of the computer that performs operations for a process.
One of the simplest algorithms is called round-robin (RR), where time slices of equal size are assigned to each process. As it is a very simple method, it handles all processes without priority. The principle behind the round-robin scheduling algorithm is simple: each running process gets an equal slice of CPU time, for example 10 milliseconds. Thus, if you have for example five processes running at the same time, each will get 0.01 seconds of CPU time, and has to wait 0.04 seconds for their next turn.
This is of course a very simple scheduling method, and modern-day operating systems are using more sophisticated scheduling algorithms, let alone multiple processors. However, even today, having a number of unnecessary processes run on your computer can still slow down your system and make your work take longer as you have to wait more for your program to respond. That's why it is crucial to keep track of your processes and stop the unnecessary ones.