• The *p = 0 adds the null terminator for the string. Same as *p = '\0';

• The comma operator is described here. https://en.wikipedia.org/wiki/Comma_operator

• the value of the comma line is whatever the last expression evaluated to (in this case it is string)

So effect is that

return *p = 0, string;

will be same as:

*p = '\0';
return string;