加粗的那几句话不能理解，帮忙解释下，谢谢了啊

#include <iostream.h>
#include <string.h>

void convto(char *s,int n,int b)
{
    char bit[] = {"0123456789ABCDEF"};
    int len;
    if(n == 0)
    {
        strcpy(s,"");
        return;
    }
    convto(s,n/b,b);
    len = strlen(s);
    s[len] = bit[n%b];
    s[len+1] = '\0';
}

void main()
{
    char s[80];
    int i,base,old;
    cout << "请输入10进制数：";
    cin >> old;
    cout << "请输入转换的进制：";
    cin >> base;
    convto(s,old,base);
    cout << s << endl;
    cin.get();
}