新手请教下各位前辈，下面这段代码的详细意思，谢谢!

function set_city(province, city)
{
    var pv, cv;
    var i, ii;    pv=province.value;
    cv=city.value;    city.length=1;if(pv=='0') return;
if(typeof(cities[pv])=='undefined') return;    for(i=0; i<cities[pv].length; i++)
    {
       ii = i+1;       city.options[ii] = new Option();
       city.options[ii].text = cities[pv][i];
       city.options[ii].value = cities[pv][i];
    }}