It is creating a histogram of the characters 0-9. It subtracts '0' which turns the values in 0-9 for the array reference, then it increments that location in the array. Each position in the array corresponds to a character.

So 0123456789 should create an array of all ones. 0123333 should create an array starting with the values 1114.

It is creating a histogram of the characters 0-9. "c- '0'" turns the value from getchar() into an integer, which acts as the index for the array. This index corresponds to the numbers 0-9. It then increments that array location. Thus, once it has completed running, the array consists of the repetitions for the characters 0-9.

So 0123456789 should result in an array of all ones. 0123333 should result in an array with the values 1114000000.