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