It’s part of the ATX Power supplyz standard. The -12V was to deal with RS232 Voltage levels as far as I know, and isn’t really used much anymore. If you meant more generically “negative volts? what’s that?”, voltage is a potential difference, so the difference between +12V and GND is 12V, and the difference between GND and -12V is 12V, while the difference between +12V and -12V would be 24V. A negative voltage can be useful as a reference if a signal you’re dealing with swings around ‘0V’, sometimes positive, sometimes negative, so that you can use, for example, an opamp to figure out what it’s doing.
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