A Time-Locked Dimension is a type of dimension that exists as a portrayal of a certain time period. A Time-Locked Dimension does not necessarily contain sentient beings that would be free to traverse through dimensions such as normal humans would, but they are locked into whatever time they are portrayed in.
Imagine as if the 21st (or even the 80th) century were forever locked in its time period -- if we traveled to this dimension now we would see it as it is the same way if we had traveled to it millions of years later.
A Time-Locked Dimension is essentially an off-shoot energy pattern, possibly created by the AEONE for purposes unknown. A Time-Locked Dimension may repeat itself over and over, perhaps with different outcomes. It is possible that it is an experiment by the AEONE to see how different the same period of time could be had even the most minuscule change had happened.
Once a Time-Locked Dimension is discovered, it is banned from all navigation systems from ever entering again, and autopilot takes over and will transfer you out as soon as possible. However, that doesn't stop some rebellious groups from visiting these dimensions. It is unknown if altering or manipulating these dimensions would have any outcome on the Core Universe.