What’s the downside with these strains of code?

i’m attempting so as to add some content material within the store web page of woocommerce. Simply tried these codes…

<?php

add_action('woocommerce_archive_description' , 'pattern' );

perform pattern()
{
    echo "Hello World";
}

?>

However Not in a position to see the specified outcome.

Is there something that I’m doing flawed??