请教一下btn += "<span></span>"

Question

//以下代码添加数字按钮和按钮后的半透明条，还有上一页、下一页两个按钮
var btn = "<div class='btn'>";
for(var i=0; i < len; i++) {btn += "<span></span>";}
btn += "</div>";
btn +="<div class='preNext pre'></div>"+"<div class='preNext next'></div>"+
"<span class='hidden left'></span>"+"<span class='hidden right'></span>";
$("#focus").append(btn);

实在是搞不懂btn += "<span></span>"和btn += "</div>"的意思，求大神解答，多谢多谢