The power supply in a PC supplies various voltages to internal devices in a computer via power connectors. These voltages don't have to be exact but they can only vary up or down by a certain amount, called a tolerance.
If a power supply is providing the parts of a PC with a particular voltage outside of this tolerance, the device(s) being powered may not work properly or at all.
Below is a table listing the tolerances for each power supply voltage rail according to Version 2.2 of the ATX Specification (PDF).
Note: To help when testing a power supply, I've also calculated the minimum and maximum voltages using the tolerances listed. You can reference my ATX Power Supply Pinout Tables list for details on which power connector pins supply which voltage.
Power Supply Voltage Tolerances (ATX v2.2)
|Voltage Rail||Tolerance||Minimum Voltage||Maximum Voltage|
|+3.3VDC||± 5%||+3.135 VDC||+3.465 VDC|
|+5VDC||± 5%||+4.750 VDC||+5.250 VDC|
|+5VSB||± 5%||+4.750 VDC||+5.250 VDC|
|-5VDC (if used)||± 10%||-4.500 VDC||-5.500 VDC|
|+12VDC||± 5%||+11.400 VDC||+12.600 VDC|
|-12VDC||± 10%||-10.800 VDC||- 13.200 VDC|