Wordpress - How can I get only parent terms?

Yes, just pass in the parent parameter to get_terms when you call it, as Michael pointed out.

Since WP 4.5 this is the recommend usage:

$myterms = get_terms( array( 'taxonomy' => 'taxonomy_name', 'parent' => 0 ) );

Prior to WP 4.5 this was the default usage:

$myterms = get_terms( 'taxonomy_name_here', array( 'parent' => 0 ) );

Will return all terms that have a parent value of 0, ie. top level terms.

use the 'parent' parameter:

http://codex.wordpress.org/Function_Reference/get_terms

or

http://codex.wordpress.org/Function_Reference/get_categories

for woocommerce email templates use the following:

$terms = get_the_terms( $_product->id , 'product_cat');
    if($terms) {
        foreach( $terms as $term ) {
            $term = get_term_by("id", $term->parent, "product_cat");
            if ($term->parent > 0) {
                $term = get_term_by("id", $term->parent, "product_cat");
            }
            $cat_obj = get_term($term->term_id, 'product_cat');
            $cat_name = $cat_obj->name;
        }
    }
echo '<br />('. $cat_name . ')';