'dtr' means 'double treble (triple) crochet'
Yarn over hook three times, then insert hook into next stitch.
Yarn over hook and draw yarn through stitch - five loops on the hook.
Loop yarn over hook and draw through two loops, (there are now four loops remaining on the hook), *yarn over hook and draw through two loops**, repeat * to ** twice more.
This completes one double treble crochet.