アクティブな状態を意味していると思います。フィドル。
.button, #rcp_submit {
display: inline-block;
border: 0;
border-radius: 0;
outline: 0;
background: #4e68c7;
box-shadow: 1px 0px 1px #203891, 0px 1px 1px #3852b1, 2px 1px 1px #203891, 1px 2px 1px #3852b1, 3px 2px 1px #203891, 2px 3px 1px #3852b1, 4px 3px 1px #203891, 3px 4px 1px #3852b1, 5px 4px 1px #203891, 4px 5px 1px #3852b1, 6px 5px 1px #203891;
color: white;
white-space: nowrap;
font-family:'Gotham Rounded A', 'Gotham Rounded B', "proxima-nova-soft", sans-serif;
padding: 9px 16px;
line-height: 1.4;
position: relative;
top: -5px;
}
.button:hover, .button:focus, #rcp_submit:hover, #rcp_submit:focus {
color: white;
background: #3d57b4;
}
.button:active, #rcp_submit:active {
box-shadow: 1px 0px 1px #203891, 0px 1px 1px #3852b1, 2px 1px 1px #203891, 1px 2px 1px #3852b1, 3px 2px 1px #203891;
-webkit-transform: translate(3px, 3px);
-moz-transform: translate(3px, 3px);
-ms-transform: translate(3px, 3px);
-o-transform: translate(3px, 3px);
transform: translate(3px, 3px);
}