c-'0' is technique to give int value == to char number e.g. 1 for '1' , 5 for '5'.
char symbols '0', '1', '2' ..... '9' are assigned continue encoding values so difference of a numeric char constant with '0' gives decimal number. (in your compiler for example in ASCII char they are assigned continues acsii values).
So for example in variable c is '7', then c - '0' == 7;
In your code array declared as:
int ndigit[10]= {0};
// default initialized with `0`
So index can be from 0 to 9. So in you code:
++ndigit[c-'0']; // ndigit[c-'0'] = ndigit[c-'0'] + 1;
increments frequency of a number by 1 when at corresponding digit of a number char.
