Multitasking is common to all WIMP operating systems (such as Windows) and is what makes them so useful. What it means is that you can run several programs at once. If you look at the bottom of the screen, you can see one or possibly more than one thing looking something like this: If there are several, you can move from one task to the other by clicking on them.
Often, you will want to move information (such as some writing) from one program to another. This can be done using the Clipboard. If you want to see two windows at the same time, restore them both and then resize them, as described in structure of a window.
You may have problems if you try to run too many programs at once. This is especially true if you are using an old computer. This is because the computer can only hold a limited amount of information in RAM and you get into trouble if you try to use more RAM than you have. Windows actually has a clever solution to this, which is to save pieces of RAM to your hard disk when they are not immediately needed. This allows you to run more programs, or more memory-intensive programs than you would be able to otherwise, but reading from and writing to the hard disk takes time. You will notice your computer slows down considerably.