没看懂“numLen - 2 - i”的含义，高手帮帮忙

import java.util.Arrays;

public class Num2Rmb{
private String[] hanArr = {"零","壹","贰","叁","肆","伍","陆","柒","捌","玖"};
private String[] unitArr = {"拾","佰","仟"};

    private String[] pide(double num){
        long zheng = (long)num;
        long xiao = Math.round((num - zheng) * 100);
        //把整数转换为字符串，两种方法
        return new String[]{zheng + "", String.valueOf(xiao)};
    }

    private String toHanStr(String numStr){
        String result = "";
        int numLen = numStr.length();
        for(int i = 0; i < numLen; i++){
            //把char类型数字转换成int，因为他们的ASCII码值恰好相差48
            //因此把char型数字减去48得到int型数字，例如'4'被转换成4。
            int num = numStr.charAt(i) - 48;
            //如果不是最后一位数字，而且数字不是零，则需添加单位(千百十)
            if(i != numLen - 1 && num != 0){
                result += hanArr[num] + unitArr[numLen - 2 - i]; // 没搞懂这个 numLen-2-i
            }
            else{
                result += hanArr[num];
            }
        }
        return result;
    }

    public static void main(String[] args){
        Num2Rmb nr = new Num2Rmb();
        System.out.println(Arrays.toString(nr.pide(236711125.123)));
        System.out.println(nr.toHanStr("6109"));
    }   

}