You can't concentrate on more than What's six times nine? one thing at once. You won't get very far reading this book if someone is interrupting you every five seconds asking you to do arithmetic problems. But any computer with a modern operating system can do many things at once. More precisely, it can simulate that ability by switching very quickly back and forth between tasks.
In a multitasking operating system, each program, or process, gets its own space in memory and a share of the CPU's time. Every time you start the Ruby interpreter, it runs in a new process. On Unix-based systems, your script can spawn subprocesses: this feature is very useful for running external command-line programs and using the results in your own scripts (see Recipes 20.8 and 20.9, for instance).
The main problem with processes is that they're expensive. It's hard to read while people are asking you to do arithmetic, not because either activity is particularly difficult, but because it takes time to switch from one to the other. An operating system spends a lot of its time as overhead, switching between processes, trying to make sure each one gets a fair share of the CPU's time.
The other problem with processes is that it's difficult to get them to communicate with each other. For simple cases, you can use techniques like those described in Recipe 20.8. You can implement more complex cases with Inter-Process Communication and named pipes, but we say, don't ...