To perform a cold boot (also called a "hard boot") means to start up a computer that is turned off. It is often used in contrast to a warm boot, which refers to restarting a computer once it has been turned on. A cold boot is typically performed by pressing the power button on the computer.
Both a cold boot and warm boot clear the system RAM and perform the boot sequence from scratch. However, unlike a cold boot, a warm boot may not clear all system caches, which store temporary information. Additionally, a cold boot performs a "power on self test" (POST), which runs a series of system checks at the beginning of the boot sequence.
While a warm boot and cold boot are similar, a cold boot performs a more complete reset of the system than a warm boot. Therefore, if you are troubleshooting your computer, you may be asked to turn off your computer completely and perform a cold boot. This makes sure all temporary data is wiped from your system, which may help eliminate issues affecting your computer.