Treble crochet (abbreviated trc) is a one of the basic crochet stitches.

At all crochet schemes treble crochet symbol would appear as:
At the scheme below you can see a stitches of treble crochet
Treble crochet instructions
Wrap the yarn around the crochet hook from back to front twice and insert hook in the last stitch of the previous row under top two loops.
Pull the yarn back through the loop. You should now see four loops of yarn on your hook. Catch the yarn.
Draw the yarn through the first two loops on the hook.
Bring the yarn over the crochet hook from back to front and draw through the next two loops on the hook.
Again, bring the yarn over the crochet hook from back to front and draw through the last two loops on the hook.
Repeat the process inserting hook in the following stitches of the previous row under top two loops.
Video tutorial
Guide to Treble Crochet in the Back Loop Only