An operating system (or "OS") is a set of programs that controls a computer. It controls both the hardware (things you can touch - such as keyboards, displays, and disk drives) and the software (application programs that you run, such as a word processor).
Some computers have a single-user OS, which means only one person can use the computer at a time. Many older OSes (like DOS) can also do only one job at a time. But almost any computer can do a lot more if it has a multiuser, multitasking operating system like UNIX. These powerful OSes let many people use the computer at the same time and let each user run several jobs at once.
UNIX was invented almost 30 years ago for scientific and professional users who wanted a very powerful and flexible OS. It's been significantly developed since then. Because UNIX was designed for experts, it can be a bit overwhelming at first. But after you get the basics (from this book!) you'll start to appreciate some of the reasons to use UNIX: