Permutations are of huge importance in computer science for the analysis of algorithms. A permutation of objects is an arrangement of those objects in a row. This leads to the concept of factorials which is the product of a given number and all the natural numbers below that to the number one.
Knuth notes that it is helpful to remember that 10! is equal to 3,628,800, which represents an approximate dividing line between things that are practical to compute and and things that are not.