Watch a video of the stream they explain it. Also it is on the weekly update. Here is the basics. You infuse a higher attack rated weapon (must be year 2) with a lower attack rating level weapon and it brings the lower level weapon up to the higher level. You will lose your higher level weapon used to infuse the lower level one though.

Thanks. Just wasn't sure if they were going to make us use Etheric Light to ultimately max the new legendaries. So basically I can just use all of my Etheric Light now since they will be useless once TTK releases