JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>

<html>
    <head>
        <script type="text/javascript">
            function popup() {
                document.getElementById('pop').style.display = 'block';
            }
            function popdown() {
                document.getElementById('pop').style.display = 'none';
            }
        </script>
        
        <style type="text/css">
            #button {
                height: 20px;
                width: 200px;
                text-align: center;
                background: #ada;
                border: 1px solid #aaa;
                border-radius: 5px;
            }
            #pop {
                display: none;
                width: 200px;
                height: 200px;
                border: 1px solid red;
                background: yellow;
                opacity: 0.2;
                border-radius: 10px;
                position: relative;
                top: 5px;
                left: 100px;
            }
        </style>
    </head>
    
    <body>
        <div id="button" onmouseover="popup();" onmouseout="popdown();">
            Наводим сюда
        </div>
        
        <div id="pop">
            Видим это
        </div>
    </body>
</html>