what does x-'a' mean? where x is another digit?

I am looking at a code where the loop iterates until the parameter is table[digits[index]-'0'].size(). What exactly does subtracting by '0' do here?

I tried printing the parameter but I get the same answer as printing without subtracting 0.Here is the code I was looking at.

class Solution {
public:
    vector<string> letterCombinations(string digits) 
    {
        vector<string> res;
        if(digits.size()==0) return res;
        string local;
        vector<vector<char>> table(2,vector<char>());
        table.push_back(vector<char>{'a','b','c'}); // index 2
        table.push_back(vector<char>{'d','e','f'}); // 3
        table.push_back(vector<char>{'g','h','i'});
        table.push_back(vector<char>{'j','k','l'}); // 5
        table.push_back(vector<char>{'m','n','o'});
        table.push_back(vector<char>{'p','q','r','s'}); // 7
        table.push_back(vector<char>{'t','u','v'});
        table.push_back(vector<char>{'w','x','y','z'}); // 9

        backtracking(table,res,local,0,digits);
        return res;
    }

    void backtracking(const vector<vector<char>>& table, vector<string>& res, string& local, int index, const string& digits) {
        if(index==digits.size())
            res.push_back(local);
        else
            for(int i=0;i<table[digits[index]-'0'].size();i++) {
                cout<<"digit: "<<digits[index]<<endl;
                cout<<"digit2: "<<(digits[index]-'0')<<endl;
                local.push_back(table[digits[index]-'0'][i]);
                backtracking(table, res, local, index+1, digits);
                local.pop_back();
            }
    }
};