'p' is a mathematical binary infix operator. (n p r) is the number of strings of length r which can be made with n different characters, and no character appears twice in the string. (It can also be written P(n, r), or nPr, or even nPr.)
Both n and r must be nonnegative integers, and r must be less than or equal to n. For example, (4 p 2) is the number of strings with 2 different elements which can be made from 4 characters. Say the 4 characters are w, x, y, and z. The possible strings are: wx, wy, wz, xw, xy, xz, yw, yx, yz, zw, zx, and zy; (4 p 2) is 12. (n p r) equals (n! / (n - r)!).
N-wing tells me maybe the infix 'p' should be capitalized, (n P r). He also says it stands for 'permutation', like c (or C) stands for 'combination'.