Evaluating a postfix expression

Question

This is a program to evaluate a post-fix expression using stack. Why in the 8th line we have pushed question[i]-'0' rather than just question[i]. I did not understand the role of 0.

stack<int> s;
int main(){
  string question;
  cin>>question;

  for(int i = 0 ; i < question.length() ; i++){
    if(isdigit(question[i]))
      s.push(question[i] - '0');
    else{
      int a = s.top();
      s.pop();
      int b = s.top();
      s.pop();

      if(question[i]=='+') s.push(a+b);
      else if(question[i]=='-') s.push(a-b);
      else if(question[i]=='*') s.push(a*b);
      else if(question[i]=='/') s.push(a/b);
    }
  }
  cout<<"Converting to infix and solving we get : "<<s.top();
  return 0;
}