Thread: Help understanding code

I found this code on the internet and I don't understand whats the point of this line : (str[i] - '0')

I know its not working without it but whats the point of subtracting 0 of something?

Code:

#include <string>
#include <iostream>
using namespace std;


int string2sec(const std::string& str)
{
    int i = 0;
    int res = -1;
    int tmp = 0;
    int state = 0;

    while(str[i] != '\0')
    {
        // If we got a digit
        if(str[i] >= '0' && str[i] <= '9')
        {
            tmp = tmp * 10 + (str[i] - '0');
        }
        // Or if we got a colon
        else if(str[i] == ':')
        {
            // If we were reading the hours
            if(state == 0)
            {
                res = 3600 * tmp;
            }
            // Or if we were reading the minutes
            else if(state == 1)
            {
                if(tmp > 60)
                {
                    return -1;
                }
                res += 60 * tmp;
            }
            // Or we got an extra colon
            else
            {
                return -1;
            }

            state++;
            tmp = 0;
        }
        // Or we got something wrong
        else
        {
            return -1;
        }
        i++;
    }

    // If we were reading the seconds when we reached the end
    if(state == 2 && tmp < 60)
    {
        return res + tmp;
    }
    // Or if we were not, something is wrong in the given string
    else
    {
        return -1;
    }
}

int main()
{

    std::cout << string2sec("01:01:05") << std::endl;
//    std::cout << string2sec("1:01:01") << std::endl;
//    std::cout << string2sec("1:2:0") << std::endl;
//    std::cout << string2sec("10:10:10") << std::endl;
    return 0;
}