What is (ch - '0')? C++

I have a Parse program that parsing arithmetic expressions in the expression class there is a function parse here is it;

//function parse
void express::parse(){
    char ch;
    char lastVal;
    char lastop;

    for(int j=0;j<len;j+=1){
        ch = pStr[j];

        if(ch>= '0' && ch <= '9'){
            s.push(ch - '0');
        }else if(ch =='+' || ch == '-' || ch == '/' || ch == '*'){
            if(s.getTop() == 1){
                s.push(ch);
            }else{
                lastVal = s.pop(); // number
                lastop = s.pop(); // operator
                //if ch is * or /  && lastop + or -
                if((ch == '*' || ch == '/') && (lastop == '+' || lastop == '-')){
                    s.push(lastop);
                    s.push(lastVal);
                
                }else{
                    switch(lastop){
                    case '+': s.push(s.pop() + lastVal);break;
                    case '-': s.push(s.pop() - lastVal);break;
                    case '*': s.push(s.pop() * lastVal);break;
                    case '/': s.push(s.pop() / lastVal);break;
                    default: cout << "Unkowen Number"; exit(1);break;
                    }//end switch
                }//end if
                s.push(ch);
            }//end if 
        }//end if
    }//end for
};

and the s.pop() that is a stack class to hold the character whether it a number or an operator and to help me make the operation first of the '*' , '/' so I need to know what is the that statement means s.push(ch - '0');? is it about the ASCII code or what i don't know but when I erase it the result change.