<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.sidebar{
position: relative;
width: 100px;
height: 100px;
background-color: red;
left: 0;
transition: 1s;
}
@media only screen and (max-width:800px) {
.sidebar{
left: -192px;
}
}
</style>
</head>
<body>
<div class="sidebar"></div>
</body>
</html>
в чем сложность?