The Time Spiral was a mysterious phenomenon located in the Time Vortex. The Time Spiral was located somewhere near the end of the Humanian Era, which is probably why Time Lords didn't go past that period in time.

In their adventure on Barclow, Romana mentioned that study of the history of this era was also forbidden; these two facts are likely connected, but it is unclear whether the Time Spiral was the cause of this prohibition, or something put in place to enforce it.

The Time Spiral could rip apart a TARDIS and anyone inside if it came too close. To deal with this risk, each TARDIS had a device fitted that caused it to materialise any time it came too close to the Time Spiral. However, the Doctor removed his at some point.

The Fourth Doctor, Romana, and K9 were nearly destroyed by the Time Spiral. (PROSE: The Well-Mannered War)

Apparently, the Time Spiral was somehow removed or destroyed either before or during the Last Great Time War, as the Master was able to flee to the end of the universe during the War, far beyond the Time Spiral. (The Tenth Doctor also travelled there after the end of the War, which is how the Master was discovered.) (TV: Utopia)