An attacher is a bit of code that can be linked to specific actions of the character sheet, like attacks, saving throws or ability checks.For instance, a rogue can set up their character so that every time they roll an attack, the chat outcome has a link at the bottom, which can be clicked to run the Sneak Attack feature. It can be used for Paladin smites, Battle Master maneuvers, Barbarian rage reminders, etc. It's a great time saver. scroll down to the bottom of any Shaped character sheet and there is a link to the documentation. Once you are in that, find the section on attachers. Also, some googling will reveal some great how youtube videos on Roll20 features, including one on attachers.