SASS Parent Selector

Parent selector is a special type of selector in SASS, which allows us to reuse the outer(Parent) selector in an efficient way. See the example below to get the idea: 

For example: Suppose we have following CSS style block,

a {
    text-decoration: none;
    display: inline-block;
    background-color: lightgray;
}

Now, You want to add a hover effect to the anchor block, then you will write

a:hover {
    background-color: gray;
}

Using SASS you need not repeat the outer selector(a), either you can do the following: 

SASS file:

a {
    text-decoration: none;
    display: inline-block;
    background-color: lightgray;

    &:hover {
        background-color: gray;
    }
}

Here & is called parent selector. Compiled CSS file will be same as our CSS style. 

Compiled CSS file:

a {
    text-decoration: none;
    display: inline-block;
    background-color: lightgray;
}

a:hover {
    background-color: gray;
}

Advantages:

1. You can create new classes with prefixes or suffixes as the name of the parent selector, which enables us to write very structured CSS styling, very efficiently. 

SCSS file:

.main-block {
    border: 1px solid black;
    position: relative;

    &_subblock1 {
        position: absolute;
        display: block;
    }

    &_subblock2 {
        position: absolute;
        display: inline-block;
    }

}

Compiled CSS file:

.main-block {
    border: 1px solid black;
    position: relative;
}

.main-block_subblock1 {
    position: absolute;
    display: block;
}

.main-block_subblock2 {
    position: absolute;
    display: inline-block;
}        

You can use different pseudo-classes or pseudo-elements to add styles in the outer(parent) selector. See in the first example we have used :hover pseudo-class to add effect in an anchor tag with the parent selector.