We don't know. It's a secret. All we really know is this:
- The Timelords & Gailfrey were sent to another universe at the end of "The Day of the Doctor" & if they return, many races fear the Last Great Time War will begin again.
- The Silence have dedicated themselves to preventing the Time Lords return.
- Since only a very few people know the Doctor's true name, the Time Lords are using it as their indicator that they have located their own universe again. Please watch "The Day of the Doctor" or "The Time of the Doctor" or see any hyperlink above for general background.
- If the Doctor speaks his true name, the Time Lords will return, most likely causing the Wars to begin anew.