linear-gradient
background: -moz-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
background: -webkit-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
background: -o-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
background: -ms-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
background: linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
请问:这里的参数50%是什么含义?