A Lightning talk is a short presentation, no longer than 5 minutes. Unlike other presentations at the OpenStack Summit. the lightning talks are unstructured and can be about anything: from code, to running, to any hobby you may have. You can use slides but the 5 minutes need to take into account setting up of your equipment.
You sign up for giving the talk the same day you'll want to deliver it. Participate to the opening sessions every day for more details.
Be creative and have fun.