The Ampersand in Sass is very useful, especially when using something like BEM syntax. It outputs the immediate parent class of the current element.
.block {
    border: 1px solid darkgray;
    
    &__content {
        background: white;
    }
    
    &__title {
        color: blue;
    }  
    
    &__footer {
        background: gray;
    }  
    
}
This code outputs the following:
.block {
    border: 1px solid darkgray;    
}
.block__content {
    background: white;
}

.block__title {
    color: blue;
}  

.block__footer {
    background: gray;
}
But what if we want to select the grandparent class, not the immidiate parent? In the following example we want the .block__title to turn red when the user hovers over the .content element:
.block {
    border: 1px solid darkgray;

    &__content {
        background: white;
        
        &:hover &__title {
            color: red;
        }
        
    }
    
    &__title {
        color: blue;
    }
    
}
The output code is not what we really want:
.block {
    border: 1px solid darkgray;    
}

.block__content {
    background: white;
}

.block__content:hover .block__content__title {
    color: red;
}

.block__title {
    color: blue;
}
What we want is to output .block__content:hover .block__title. Sass does not let us do that with its built-in tools, but we can do something else – use variables.
.block {
    $blockClass: &;    
}

Now the variable $blockClass is set to .block and we can reuse its value further.

.block {
    $blockClass: &;
    border: 1px solid darkgray;

    &__content {
        background: white;
        
        &:hover #{$blockClass}__title {
            color: red;
        }
        
    }
    
    &__title {
        color: blue;
    } 
    
}

#{$blockClass} just outputs the value of $blockClass variable.

.block {
    border: 1px solid darkgray;    
}

.block__content {
    background: white;
}

.block__content:hover .block__title {
    color: red;
}

.block__title {
    color: blue;
}