A time loop/ temporal loop is a common plot device in science fiction (especially in universes where time travel is usual) in which time runs normally for a set period, but then goes back like a broken record. When the time loop "resets", the memories of most characters are reset (i.e. they forget all that happened).
Time loop in Tsubasa Reservoir Chronicle
Physical Time Loop
In a physical time loop, the spacetime loops around to form several closed timelike curves. Since the time in that region is looped, you could only escape it by leaving the affected area. Also, there would be an infinite number of copies of any matter in the area, unless an object left the loop. In that case, there would only be as many copies of that object as many times it completed the loop. This type of time loop cannot be ended or destroyed.
This is seen when the Clones are reborn and give birth to Tsubasa Li. Sakura and Syaoran Li realize
that their lives are forever condemned to live in a eternal time-loop, and thus time will be repeating itself over and over again, like a circle without end. So, they decide to seal themselves inside Clow Reed's tube, and break the cycle once they meet Fei Wong and restore the logic of the universe.
Tsubasa Li also experienced this in Clow Country (Original Timeline). Clow had been sealed in a time-loop, because every day was Sakura's birthday. Time just continued to repeat itself again and again, and each time people's lives were lost. Exception for this time-loop were given to Sakura and the Royal Family. Fei Wong, Xing Huo and Kyle Rondart also habited Clow unharmed by this loop.
This time-loop was also restored in the last arc.