Straight out of cannabis.com:


If you've stuck with me this far, you are wondering what a "fan" leaf is. They are the wonderful, fat, shade leaves that are going to cut the light to all of the bud sites developing down thru the plant and must be removed.

"Snip the fan leaf stem close to but not touching the node. Remove the upper fan or shade leaves to expose the floral clusters. Do not trim all the leaves away for the upper portions of the plant as they are still necessary for food production."