Fletching is a technology that can be researched at an Alchemy Lab. Every upgrade of Fletching increases your projectiles' range by 5%. It also allows you to train Archers and build Wall-Mounted Crossbows.
To view a detailed information for the required Resources per research level please view the Research page.
Fletching is probably the most important research but you have to get lvl 4 Logging to be able to start researching it.
High level fletching is required to attack wildernesses and barbarian camps.