A buzzer is a little component attached to motherboard of a computer system.
A buzzer is essentially a small speaker that produces high frequency buzzing sounds. Signals to produce the sound come from BIOS of the related computer system.
A buzzer is capable of producing several kind of high pitch sounds to inform or warn the user of the status of the system.
The buzzing produced is created after the turning on or restarting the system. Buzzers produce sound immediately before the operating system starts and the user, if familiar with buzzer signals, understands that everything is fine or there is a problem with the system.
It is worth noting that not all computer systems contain buzzer.