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 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 prefix or suffix 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;
   }
   
           

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