A Timeslip is an enclosed area where Time can jump back and forth, into the past and the future and eveything in between.
Timeslips are mostly random occurences in the Nightside. Few understand how they work, why they appear, or what causes them.
The Authorities have done extensive research into the phenomena and managed to create a portable Timeslip for Walker's use. With it, he can be where he needs to be when he needs to be there, and oftentimes before that.