Assuming the position field is filled with numbers to give order: have a look at ksort
.
<?php
$terms = get_terms('brands');
// Let's create our own array and then reorder it
$order_terms = array();
foreach( $terms as $term ) {
$position = set_up_the_position_meta_here;
$order_terms[$position] ='<h2>'.$term->name.'</h2>';
}
// now lets reorder the array based on keys (position)
ksort($order_terms);
// time to display
foreach( $order_terms as $order_term ) {
echo $order_term;
}
?>