This stitch is equivalent to 5 ch sts.
Its abbreviation is "dtr" and its symbol is this one:
These are the instructions to make a Double Treble:
Yarn over hook 3 times, insert crochet hook in st (the 6th ch from the hook if it's the 1st one on the foundation chain), yarn over, pull through st, [yarn over, pull throug 2 loops on hook] 4 times.
Here's is a video tutorial: