To reboot is to restart a computer and reload the operating system. The most common reasons to reboot are because the installation of new software or hardware requires it, or because applications are not responding for some reason. On computers running Windows, you can usually reboot by selecting "turn off computer" from the start menu and then clicking "restart" in the window that pops up. Another way (and one that works sometimes when the first way doesn't) is through the Ctrl-Alt-Delete keystroke combination, which was developed as an easy way to reboot a computer that would nevertheless be an unlikely accidental keystroke combination.
Rebooting a computer through the menu option or the keystroke combination is sometimes referred to as a warm boot, perhaps because it is more gentle than the alternative cold boot (simply pressing the computer's power button once to turn it off and then again to turn it back on).