function(x)中的x是代表什么？

Question

for (var i=0; i < document.getElementsByClassName('gg_blank').length; i++) {

    var tabletd_gg=document.getElementsByClassName('gg_blank')[i].innerHTML;    

    var count = 0;

    var res_gg = tabletd_gg.replace(/~/g, function(x){ var symbol = count++ == 1 ? '~<br/>' : x; return symbol;});

    document.getElementsByClassName('gg_blank')[i].innerHTML=res_gg;

    }

function(x){ var symbol = count++ == 1 ? '~<br/>' : x; return symbol;}
这句话咋理解啊，主要不知道x代表的是啥，我好蠢