EventHorizon displays all of your class/spec’s rotational abilities on a common time scale. This means that instead of seeing a regular percentage-based timer bar, EH shows you exactly what is happening in relation to everything else. If that sounds complicated at all, just look at the screenshots.

The basic idea: An ability that lasts 12 seconds and has 6 seconds remaining shouldn’t look exactly the same as one that lasts 5 minutes and has 2.5 minutes remaining.