You could provide your editors textarea’s ID to get the content of the tinyMCE, also you should get the contents of the saved post meta and set it as the text of your editor element to display the value saved before. You could use the following code as a reference,
<?php
function taggr_meta_function($post){
$my_editor = get_post_meta($post->ID, 'my_editor', true);
wp_editor( $my_editor, "my_editor", array( 'textarea_name' => "my_editor" ) );
?>
<script>
window.onload = function(){
alert(tinyMCE.get('my_editor').getContent());
}
</script>
<?php } ?>