What is the meaning of the “$menuHeight” in .css Files?

here below i understand that '$menuHeight' is sort of variable that actually stores certain value but why visual code is showing error for this, is it obsolete now or logically is it wrong ?

 $menuHeight: 65px+10px;
    #mazeContainer {
        transition-property: opacity;
        transition-duration: 1s;
        transition-timing-function: linear;
        **top: $menuHeight;**
        opacity: 0;
        display: inline-block;
        background-color: rgba(0, 0, 0, 0.30);
        margin: auto;
    
        #mazeCanvas {
            margin: 0;
            display: block;
            border: solid 1px black;
        }
    }