it is when you trim off the bottom 2/3's (or less) of your fan leaves so you only have close growth up untill you hit the top. The thought is that it allows more light to hit your lower flowering/budding areas thus increasing yield. some say it works like a dream, some say it is just retarded to do because you are taking away the fan leaves which are collecting the most light and feeding your plant/flowers/buds.... it's one of those things, "to each their own", do what you want and what works for you.
IMO leave the leaves unless they are more than 1/2 dead/burned