tr = treble (triple) crochet
With 1 loop on hook, wrap the working yarn 2 times; you’ll have 3 loops on your hook. Next insert hook into stitch (can be desired stitch or foundation chain) and wrap working yarn on hook and pull through. You will have 4 loops on your hook. Pull through 2 of the loops. Now wrap hook again and pull through the next two loops still on the hook. Wrap for the last time and pull through the last two loops still on the hook.
Watch Mikey demonstrate for us in the link and video below.