To perform warm boot (also called a "soft boot") means to restart a computer. It is often used in contrast to a cold boot, which refers to starting up a computer that is turned off.
A warm boot is typically initiated from the operating system, rather than pressing a button on the computer itself. For example, to perform a warm boot on a Windows system, you may select from the Start Menu. In Windows 8, you can open the Charms bar and select to reboot the computer. If you use a Mac, you can perform a warm boot by selecting from the Apple Menu.
Warm booting (restarting a computer) is more common than cold booting since most people leave their computers in sleep mode when they don't use them. While a home computer may not need to be turned off for months, it may need to be restarted every few days or weeks to complete new software installations. Even though a warm boot does not include a power on self test (POST) like a cold boot, it performs the rest of the boot sequence and therefore loads any system files that were installed before the computer was restarted.