ChaosPower shows you the current relative strength of your Chaos Bolt. This means if the number is bigger, your Chaos Bolt would do more damage if you cast it right now. So basically wait for that number to get as big as possible and then go all out with your Chaos Bolts.
For a Destruction Warlock, good DPS depend on casting those precious Chaos Bolts when they are going to do the most damage. Most guides say "stay at 3.5 embers, wait for trinket and/or weapon enchant procs, then activate Dark Soul and (and on-use-trinkets) and unload all your embers with Chaos Bolts". But in my experience there are large damage differences between different procs. This addons purpose is to make these differences visible, so that you are able to more reliably pick the very best moments for casting Chaos Bolts.
ChaosPower only has some basic configuration options (unlock, size, font size, hide out of combat and toggle the icon).
They are slash command based and are accessible via /cp or /chaospower.