Your Hyper Combo Gauge is located at the bottom of the screen and you can hold up to 5 charges at a time. It builds as you attack your opponent and you use it to unleash a Hyper Combo on them. Each character has more than one Hyper Combo move and you should be able to build this bar up at least 3 or 4 times during one match. Morrigan can also build the bar up with her Dark Harmonizer Assist. Just call her in and after about 3 assists, you'll have one full Hyper Combo Gauge bar to use.
This can be done in Arcade Mode or in an Xbox Live match.