Use for questions about returning settings to their default or original values. Other tags are more appropriate for changes on a larger scale, such as [reinstall], [restore], [system-restore], or [system-recovery], unless that's the mechanism required to restore settings.
In general, "reset" refers to returning something to its default or original state. For purposes of this site, it refers to settings. Changes on a larger scale, such as reinstalling software or restoring from a backup (unless that's the mechanism required to restore settings), or returning the system to a different software complement are covered by other tags.
There are three aspects.
There are typically settings that software references to control its operation. If these are changed, a reset returns these to predefined values.
Some settings may be read only when the software is loaded, so changing the settings won't immediately change how the software functions. In this case, the software needs to be restarted for the change in settings to take effect.
Some software comes originally with "factory settings" but provides for customization. From either starting point, the settings might be temporarily changed to apply to that session, or the loaded values might become corrupted. There are two "levels" of reset:
Soft reset generally involves just restarting the software to clear temporary changes or corruption and reload the default settings.
Hard or factory reset involves changing the default settings back to their original (factory) values. How this is accomplished varies. There might be a protected reset switch or special combination of buttons, or it might require reinstalling software or re-flashing firmware.
The term "software" is used here in the very generic sense. It could be a program, operating system, firmware, or in some cases, even electronic logic or electro-mechanical components that have stored voltages or entered a state that causes improper operation.