Zimmerman is, or was, or on some profound level may still be, a History Monk who made great breakthroughs in the practical art of slicing time. Lu-Tze refers to him with admiration as one of the very best there was, and we can infer greatness from this one reference alone, as Lu-Tze is not easily impressed.
He is credited with going deeper and further into Time than any history monk before him, with the exception of Wen himself. Zimmerman could slice Time into thinner and thinner slices, passing through regions of great temporal instability until he reached the deep, but stable, zone known to the Monks as "Zimmerman's Valley" in his honour. Once in the Valley, a monk who can stand the extremes of cold and scarcity of useable oxygen is capable of travelling further and faster than ever before. Use of the Valley, in fact, is how Lu-Tze and Lobsang Ludd got to Ankh-Morpork from the Hub in just enough time to witness the switching-on of the Glass Clock.
Zimmerman predicted that there would be another stable region even further down the Time Slice than the Valley. According to Lu-Tze, he was doing just fine looking for it, right up until the moment he exploded. But in the final desperate spurt into Ankh-Morpork, Lobsang Ludd found this previously undiscovered area with little additional effort and would have got to the clock in time to destroy it, but for the fact that Lu-Tze was outstripped by his apprentice and showed signs of distress (he collapsed bleeding from the ears). Ludd turned back to help him, and lost his opportunity.
"Zimmerman" has several referents on Roundworld. One is to poet-philosopher-singer Bob Dylan, who was born Robert Zimmerman. At least two quantum physicists are called Zimmerman. See: Talk:Slicing time
The stable zone after instability might also refer to the hypothetical stable zone in the periodic table, right after the most unstable elements known to man.