This circuit is part of the TARDIS's Main Space-Time Element.
TARDISes are built not to change history and this circuit is designed to prevent the operator from breaking the First, Second,
Third, and Fourth Laws of Time.
These protocols are not physical laws per se, but rather something built into the Governing Circuits of all Gallifreyan time travel technology.
The Third Law of Time prevents anyone from traveling into Gallifrey's past,
and ensure that a Time Lords personal time is always synchronized with Gallifrey's time. It also ensures that Time Lords will be synchronized when the meet outside of Gallifrey.
Removal of the Relativity Differentiator will allow a TARDIS to travel into Gallifrey's distant past (before the Eye of Harmony), but it will prevent the TARDIS from changing its spatial coordinates relative to its current planet.
In order for the Governing Circuit to function as it does, it's likely that TARDISes know significantly more about where they are going then their operator does. For example, they would need to know where other TARDIS and Time Lords have also traveled. To be effective, they would have to have the power to alter destinations times to ensure that Time Lords never violate the Protocols of Linearity.
Any time a TARDIS crosses its own time stream it releases chronal energy that can damage space-time. The Time Track Crossing Protection Protocol is a pre-set circuit that (in a normally functioning TARDIS) constantly prevents a TARDIS from crossing its own timestream by arriving before it left in the same general area. Some Type 40 TARDISes have faulty Protocol circuits that have unreliable connections, causing them to disengage and allow the TARDIS to break some of the Laws of Time.
It takes a major surge of power from the Prime Eye of Harmony to override this circuit (this must be authorized by the High Council).
Despite this, a clever Time Lord will find these limitations easy to avoid. For example, the Time Track Crossing Protection Protocol can be overridden (though the operator then risks causing the TARDIS to vanish into a temporal nullity).